They are voiced by the late Chuck McCann.
As their name implies, they are a gang of amoeba who were originally featured in McCracken's original short, Whoopass Stew.
They reappeared in the World Premiere Toons segment "Crime 101". With their gangster affectations, these aspiring criminals would love nothing more than to be regarded as serious villains worthy of fighting, and even getting beaten up and sent to jail by the Powerpuff Girls. Unfortunately, their brains are far too primitive to devise a crime above the level of littering or jaywalking (in contrast to the original short, where they are seen having successfully robbed a bank and using their semi-solid bodies to stick to the girls and create a dilemma for them); indeed, they considered stealing an orange to be their greatest crime ever (in the episode "Divide and Conquer").
They are definitely Townsville's least malicious villains, even having a friendship-of-sorts with the Powerpuff Girls. They are generally harmless, but their actions have on several occasions endangered both the girls and the town purely by accident. The greatest threat they have posed to Townsville and the girls was in "Geshundfight," when they each caught a cold that, when merged with their DNA, created a severe virus that quickly spread through the city, making even the healthiest person fall ill instantaneously. It wasn't until the girls, who were also infected, acquired their DNA (after a brief and very easy fight, despite the fact they were all very sick) so Professor Utonium could create a vaccine he soon dispensed to the town, curing them just as quickly.
The biggest act of villainy they've ever committed was successfully stealing every orange in town, which caused everyone to get scurvy. They were able to do this when seeing an orange split into it's segments triggered something in their minds which made them undergo Mitosis and divide to great numbers. Fortunately the girls were able to merge all the copies back into the original criminals and locked them up.
In Special Episodes
See Me, Feel Me, Gnomey
The Amoeba Boys appeared in the rock musical episode "See Me, Feel Me, Gnomey", where, like other villains, they are seen causing chaos and terrorizing people in Townsville. Yet, when it comes the Gnome, he is responsible for eliminating all the villains, including the Amoeba Boys, but later, when all the villains revive, the Gnome loses power.
They are also seen along with all the villains singing "Why Can't We All Get Along?".
The Powerpuff Girls Rule!
The Amoeba Boys appeared in "The Powerpuff Girls Rule!", trying to get the key to the world and take over Townsville. They were all seen attacking the other villains and Powerpuff Girls in attempt to get the key. During the race to get the key, The Amoeba Boys could not even get their own car so they just called a taxi. At the end of the episode, they all got beaten up.