The retromutagen or "anti-ooze" is a substance featured in Batman vs. Teenage Mutant Ninja Turtles. Upon learning that Ra's al Ghul and Shredder were using mutagen on some of the League of Assassins ninja, Batgirl and Donatello began work on a substance that could reverse the mutations. They quickly produced a retromutagen, though its effects were limited: only mutations occurring within the last twelve hours would be reversed. Furthermore, Batgirl was unsure whether the substance would have lethal side-effects.
Despite these concerns, it became necessary to test the retromutagen when Batman was transformed into a crazed mutant bat. Michelangelo was able to inject him, which immediately caused his body to revert back into human form and reversed the effect of the Joker Venom. Fortunately, Batman survived the process, and retromutagen was provided to the Gotham City police in order to turn the mutated Arkham Asylum inmates back to normal.
The retromutagen was also a vital weapon when the Ninja Turtles and Batman infiltrated the Ace Chemicals facility. They were attacked by a number of mutated ninja from both the League and the Foot Clan, and successfully demutated some of them with thrown syringes.