Mutagen, also known as Retromutagen Ooze, is a mucilaginous substance that has served as a key plot device in nearly every medium of the Teenage Mutant Ninja Turtles. Any creature that is exposed to mutagen will rapidly increase in size, strength and intelligence (sometimes), and even gain superpowers. It is responsible for mutating, among others, Splinter and the four Ninja Turtles, and villain pairings such as Bebop & Rocksteady and Tokka & Rahzar.
The mutagen was depicted slightly differently in this cartoon; rather than causing its victims to evolve into a humanoid state, it caused them to transform into a hybrid of their original species and whatever creature they have most recently been in contact with. It was revealed in the episode "Invasion of the Punk Frogs" that the mutagen can be made mixing nyotrinaline with Taxon-9. In the episode "Enter: Mutagen Man", a third ingredient was mentioned - Bindex3. In the episode "Shredder & Splintered," it's acidic. It is also responsible for creating many of the show's mutants, such as Bebop, Rocksteady, the Punk Frogs, Leatherhead, Slash, Groundchuck, Dirtbag, and Mondo Gecko, to name but a few.
The mutagen also had many color variations through the series. The one that mutated the Turtles and Splinter was purple, Bebop and Rocksteady had pinkish glowing mutagen, and Vernon and Irma were affected by a rainbow variant of the green mutagen, which was the most common in the series.
In the episode "Shredder & Splintered", it was mentioned as a neutralizer for the mutagen. Later in the series it appears to be Compound X7, a rare experimental anti-mutagen.