1. An analytical h.p.l.c. method has been developed which permits the separation and quantification of the in vitro metabolites of 4-aminobiphenyl (4-ABP). The method employs gradient elution from a reverse phase column. 2. The major metabolite in vitro of 4-ABP in liver fractions from rat, mouse, guinea-pig, rabbit and hamster was N-hydroxy-4-aminobiphenyl. 3. The observation that liver fractions from the guinea-pig are very effective in the n-hydroxylation of 4-ABP is in excellent agreement with the 1966 report from Kiese's laboratory, showing that the N-hydroxylation of 4-ABP is an important metabolic pathway in vivo in this species. 4. The ortho-phenol, 3-hydroxy-4-aminobiphenyl was also an important metabolite in each species except guinea-pig and rabbit. 5. Hydroxylation at the 4' and 2' positions was a minor pathway in all species studied. 6. Aroclor 1254 was a potent inducer of N-hydroxylation in rat, mouse and guinea-pig but not hamster and rabbit. Phenobarbital induced N-hydroxylation in rabbit and guinea-pig but not rat, while methylcholanthrene induced in rat and guinea-pig but not rabbit.