6 polycyclic aromatic hydrocarbons were assayed for mutagenicity in the Ames test, in the presence of hepatic post-mitochondrial preparations isolated from the mouse, rat, hamster, pig and man. Benzo[a]pyrene, dibenzo[a,i]pyrene and benz[a]anthracene gave a positive mutagenic response only in the presence of activation systems derived from the hamster. With the exception of the pig, activation systems derived from all animal species could convert 3-methylcholanthrene to mutagens, the hamster being the most efficient. With the exception of the rat and pig, all animal species activated 7,12-dimethylbenz[a]-anthracene to mutagens, the human preparation being the most effective followed by the hamster and mouse. Dibenz[a,h]anthracene was not activated by any of the hepatic preparations. It is concluded that, among the animal species studied the hamster is generally the most efficient in activating polycyclic aromatic hydrocarbons to mutagens in the Ames test.