A.N.: This is a request by killercroc.
Disclaimer: I don't own Bleach.
Warning: Genderbending! Don't like, don't read!

The Serum

Chapter One: The Formula

Shortly after Grimmjow's assault
Squad 12; R&D dept.

At his desk, Mayuri finished typing the last of the data onto the computer. Behind him was a small machine, similar to that of a coffee pot. Multiple vials were inserted on top, filled with chemicals of all varieties.

The door opened up and Mayuri's artificial daughter, Nemu, entered the lab with several visitors. "Master Mayuri, I've brought whom you've requested."

"Excellent. I'm almost done." Without looking up at his computer Mayuri cocked a thumb over to the sample refrigerator nearby. "Do me a favor and put insert the last compound while I finish calculating all the variables."

Behind her, Captain Byakuya Kuchiki glared at the mad scientist. "Captain Kurotsuchi, why have you summoned us? We have no time to entertain your little science experiments," he said bluntly.

Standing beside him, Kenpachi Zaraki merely huffed in annoyance. "This had better be good, Kurotsuchi. I've got better things to do than to play lab rat for you."

"Will you two barbarians shut up?!" Mayuri roared, spinning around in his chair and staring at them with angry eyes. Byakuya noticed by the red, tired look in Kurotsuchi's eyes that the man had hardly slept. "This is a very important experiment and I'm so close to finishing it. I will answer your silly questions in due time! For now, let me WORK!" He turned around and started to type madly on the keyboard, incensed by the interruption.

Sighing, Byakuya turned to Mayuri's daughter and spoke in a much quieter voice. "Lieutenant, can you tell us what this is all about?"

"Master Mayuri has developed a new formla for unlocking a Soul Reaper's increased potential," Nemu explained. "He wanted to ask for your cooperation in testing its potency. Excuse me," she said quietly, turning back to the machine.

Byakuya noticed that the vial in Nemu's hands contained dark red blood. "Who's blood is that?" he asked.

Kenpachi sniffed the air and raised an eyebrow. "Ichigo's."

Byakuya turned to his comrade with a look of confusion on his face. "How could you possibly know that?"

"I'd know the smell of his blood anywhere."

"Quite astute for a bloodhound like you, Zaraki," Mayuri noted, looking over the complex formula on the screen. "But you're correct. For years I've been developing a serum to augment a Soul Reaper's body, preferably without Hollowfication unlike Souske Aizen's methods. I'd all but given up on the final piece of the puzzle but it wasn't after I observed the boy's battle with you, Captain Kuchiki, that I realized that he was the missing piece."

"What's Ichigo got to do with this?" Kenpachi asked.

Sighing, Mayuri turned around in his chair and turned to the two men. "Please, you of all people should the boy's boundless potential. In the span of several days he became strong enough to match you two in battle. I am of the mind that the secret lies in his blood."

Byakuya scowled as he remembered something from his battle with Ichigo on top of the Sokyoku Hill. "You claim that you don't wish to increase one's strength through Hollowfication. But Ichigo already has that power."

"Yes, I'm well familiar with that." Reaching for the keyboard, Mayuri opened his digital archive and opened up a video. Byakuya stared at himself as he was attacked by Ichigo in his Hollow-possessed state. "The boy has the power of a Hollow, yes, but I believe it's more than that. After closely examining his blood I've concluded that there is another hidden element to his latent potential, one that can be exploited."

"How'd you get Ichigo's blood anyway?" Kenpachi asked.

Mayuri waved a hand dismissively, sitting back in his chair. "Please, it wasn't all that hard. Thanks to you two he left enough samples lying all over the place. Obtaining a suitable amount for testing was child's play."

Crossing his arms, Byakuya continued to scowl at the man. "So you want us to test this formula for you? Why not have your daughter test its effects?" he asked, glancing over at the silent Nemu, who was busy imputing data into the chemical mixer. "Don't you normally use her as your guinea pig?"

"Oh come now, Captain Kuchiki. I'd rather not waste the hassle of creating her all over again when I have two new subjects to test it on. Besides, Nemu is not a normal Soul Reaper. There are too many variables when considering what the formula might do to someone of her physiology."

Turning back to the computer again, Mayuri finished typing the last of the data. "There. And now…" He hit one last button then turned toward the chemical mixer. "Finished."

The machine lit up and one by one the vials were drained into its confines. Byakuya and Kenpachi watched, the latter quite bored, as the machine shook and sputtered. Soon a bright red liquid dripped into a beaker. "And done." Mayuri hopped up and took the beaker, examining it closely. "Success!" He poured the liquid into two small glasses. "Now then…" he turned back to his fellow captains. "My test subjects."

Byakuya took a step back. He'd done some things in his long life that he'd regretted but being Mayuri's guinea pig was not going to be one of them. "Why would we want to? Why not have one of your men test it?"

"I require test subjects with significant spirit energy. My men are unsuited for it. Also," Mayuri held up a finger and pointed to the image of Ichigo on the screen. "You two have a connection with Ichigo Kurosaki since you did battle with him. I believe that you would benefit the most from a concoction created with his blood."

Both men looked at each other skeptically. Even Kenpachi could see how flimsy that argument was and knew that Mayuri was just spitballing. But Kenpachi remembered the raw power that Ichigo had and couldn't deny his curiosity. And Byakuya remembered that they were at war with Aizen. They needed all the advantages they could get without stooping to Aizen's level.

The two hesitantly took the glasses and looked at them with caution. "Well what are you waiting for? Go on!" Mayuri insisted.

Byakuya smelled the formula. He raised an eyebrow when he recognized the smell. 'Strawberries?' He turned and saw Kenpachi already downing the drink like it was a glass of booze. Giving Mayuri another scathing glare he brought the glass to his lips and drank it. He was surprised by the sweet taste, the formula going down easily. He looked down at himself and felt no change after a minute. The look on Kenpachi's disappointed face told him he wasn't feeling any change, either physically or in his spiritual pressure. "It would seem that your formula was faulty," Byakuya pointed out, much to Mayuri's irritation.

Kenpachi huffed and turned around. "This was a waste of time. I can't believe I thought I'd get stronger by drinking Ichigo's blood. That's just in-" The glass slipped out of his hand as the huge man stumbled. "Urgh!"

Byakuya's eyes widened in alarm as he felt the formula's effects kick in. He felt the breath knocked out of him and he fell to his knees, the glass slipping from his hand and smashing onto the floor. He looked down at his hand and saw his skin bubble, his body changing at a rapid pace. "What…what have you done?!" he choked before collapsing to the floor. He heard Mayuri say something but it was drowned out by a huge ringing in his ears. His blood boiled as he writhed on the floor. He tried to think of something to focus in order to fight back the pain. He tried thinking of his grandfather, his wife, Renji and Rukia. But all that came to mind was Ichigo. Finally, the stoic, proud captain crumpled to the floor and screamed as his body burned, Kenpachi's roaring scream joining his.

Byakuya's mind was bombarded with strange thoughts, the heat focusing in his chest. Soon he curled up into a ball as the heat inside him stopped, only to be replaced by freezing cold. He shivered as if his veins were filled with ice. His hairpins slipped from his hair as his body continued to change, morphing him from the inside out.

Mayuri and Nemu watched as the captains stopped their shaking and their screams subsided. "Oh my, are they dead?" Mayuri wondered aloud. He'd hoped to have avoided unnecessary deaths, especially since the Head-Captain was so damn insistent that his experiments not cost the squads the lives of any captains. "Oh well, I suppose I'll have to settle with dissections to find out the formula's…eff…ects…" Mayuri's words died in his mouth as he saw Byakuya rise to his knees, panting for breath. Groaning, Kenpachi also made it back to consciousness. But when they both turned to Mayuri, the scientist realized that maybe he made a mistake with his formula.

"Oh dear. The Head-Captain is not going to like this…"

Meanwhile…
Hueco Mundo; 8th Espada's Palace

Souske Aizen's footsteps echoed as he made his way into Szayelapporo's palace. The scientist's creations all bowed as he walked past him, knowing to show their master's master due respect.

It didn't take Aizen long to find Szayelapporo. The man was right where Aizen had left him the last time he visited several days ago. In his workshop, the man was standing in front of mutilple vials of chemicals. Beside him was the same machine that was in Mayuri's lab. "How goes your pet project, Szayelapporo?" Aizen asked as he neared.

"Lord Aizen," Szayelapporo addressed. When he turned around, the despot could see the wide smirk on his face. "You'll be pleased to hear that the process has been completed." He gestured to the machine as it spewed red liquid into a beaker. "This formula you provided me has been fascinating to tinker with."

"Yes, I suspected that Mayuri's obsession with surpassing others would be a good weakness to exploit." He looked over at a desk covered with papers drawn with chemical formulas. "He believes himself a clever man. But he's more predictable than any of the others." Aizen anticipated Mayuri creating the formula for unlocking hidden potential. It's what he would have done in his shoes.

Examining the liquid, Szayelapporo gave another smirk. "A pity that Ichigo Kurosaki doesn't realize the value of his own blood. If he did, he wouldn't spill it so easily," he snickered.

"Yes. I suppose Grimmjow's truant behavior was a benefit in disguise," Aizen noted.

The blood that Aizen and Szayelapporo used was taken during Ichigo's fight with Grimmjow. There was enough blood stained on Grimmjow's jacket that Szayelapporo was able to isolate it and use it for their own purpose. "A pity he's been knocked out of the Espadas," the scientist noted. "His stupidity actually came through for us."

"Yes, which is why I believe he should be the one to test it," Aizen suggested.

"Excellent. Since he's no longer an Espada, that means he's worthless. Might I suggest Ulquiorra as a potential test subject?" the mad scientist asked. "It would be prudent to have two test subjects for comparison."

Aizen nodded, knowing that Szayelapporo was only asking because he was always waiting for the chance to experiment on his fellow Espada. "Very well. But only those two," he cautioned.

"Two will be enough."

Soon Aizen found himself sitting in the Espada's conference room. Beside him sat his scientist while Ulquiorra sat next to him. Because he was demoted from the Espadas, the one-armed Grimmjow stood by the wall, a look of irritation on his face.

Ulquiorra stared at the cup with the formula in it with an inquisitive stare. "And what will result in this, Szayelapporo?"

"I cannot give you a straight answer. This is a very complex serum. One created by a Soul Reaper, no less. I won't know what it does until I see with my own eyes."

Grimmjow scoffed at his cup on the table. "Like I need this," he grumbled.

"Come now, Grimmjow," Szayelapporo taunted, the sneer on his face evident. "Surely with one arm you'll need as much of a power boost as you can."

Clenching his teeth, Grimmjow fought the urge to attack the Espada. He turned to his lord. "Why should I drink that runt's blood?"

"You underestimate Ichigo Kurosaki, Grimmjow," Aizen pointed out. "Even Ulquiorra understands the boy's latent potential."

Ulquiorra picked up his glass and looked at the former sixth Espada. "Lord Aizen has commanded you to drink it, Grimmjow. Drink." To prove his dedication, Ulquiorra brought the drink to his lips and downed it.

"Tch!" went Grimmjow as he stormed over to the table. "Fine. But if this makes me grow a second head, just know that I don't need two arms to rip your head off, Szayelapporo!" Grabbing the cup, he chugged it and threw the glass to the floor, smashing it. "There! Happy?"

"In a minute," Szayelapporo said. "And in three…two…one…"

Grimmjow's eyes widened as a tidal wave of disorientation hit him. Ulquiorra likewise slumped back in his chair, uncharacteristically groaning in pain. Both Aizen and his scientist watched while the two began to change. Grimmjow started to scream in agony as the stump of a left arm burned, like someone was taking a red hot iron and shoving it into every nerve he still had. His eyes widened to their limit as a mass of tissue erupted from it, bubbling into a new arm and hand. Grimmjow couldn't believe it: he had his arm back!

But the celebration was cut short when pain racked his chest. "What's…going on?!" he groaned, slumping to the floor. Ulquoirra, meanwhile, slumped onto the table. His long fingernails clawed at the ebony stone as his vision blurred. Color danced across his eyes and he felt his black hair suddenly grow and grow. He looked at his bubbling hand and grimaced. "What…have you done to us, Aizen?!" he choked before flashes of a certain Subsitute filled his mind. "Urghhh!"

The blue-haired Arrancar was on his hands and knees, trying but failing to fight the pain. Suddenly, the heat in his chest transferred to his lower half. As his blue hair grew past his shoulders, he squeezed his eyes shut as he felt like his legs were melting. A gasp left his lips as the heat suddenly focused in his genitals, heat burning like he was about to explode.

Then it was over.

Aizen watched with interest as his subordinates finally stopped groaning and screaming. Ulquiorra lay on the table with his head down, taking deep breaths as the fire inside of him died out. Grimmjow lay on the floor in a daze, not sure of what just happened. After a minute of waiting, Aizen's patience was rewarded when Ulquiorra sat up. He raised an eyebrow when he saw the change in Ulquiorra. Whatever he was expecting Mayuri's formula to do, this was not it.

"Lord Aizen…" the ebony-haired Espada gaped, staring down at the changes made. "What…what have you done to us?!"

Grabbing the edge of the table, Grimmjow glared at the scientist with vengeful eyes. "Oh I'll tell you what he's done to us!" Grimmjow stood up. And unlike Ulquiorra, who wore a modest uniform, Grimmjow's open vest showed off her naked breasts. "You turned us into chicks!" the genderbended Arrancar roamed.

Ulquiorra stared down at her breasts, the large puppies straining her white shirt. "I believe that you may have made a mistake."

Meanwhile…
R Mayuri's Lab

The door burst open and Rukia, Renji, Ikkaku and Yumichika all charged into the lab. They'd been alerted by Yachiru when she told them that she saw Nemu lead Byakuya and Kenpachi into the lab. Not knowing what the mad scientist intended to do, the four quickly ran for Squad Twelve, hoping to put a stop to whatever Mayuri had in mind.

A pity they were too little too late.

Rukia saw her brother turn to her as they barged in, only to blink in shock when she saw Byakuya. "B-brother?!" she gaped, recognizing Byakuya's steely gaze.

Ikkaku frowned. "I think you mean 'Sister', Kuchiki."

Byakuya stood in front of the group a changed person, transformed into a beautiful woman. Beside her stood Kenpachi, who'd also morphed into the opposite sex. Both women turned towards the scientist with angry eyes. "Captain Kurotsuchi," Byakuya seethed, unable to believe this had happened to her. "You have some explaining to do…"

Mayuri couldn't help but chuckle. "Well, at least we know what it does."

"You turned us into chicks!" Kenpachi roared, pointing at her bandage-covered DD-cups. "You're going to pay for this!"

The four onlookers stared at the surreal image for a moment before Rukia fainted…

To be continued