When metabolically activated, 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), a heterocyclic amine isolated from cooked food, is clastogenic in cultured Chinese hamster and human cells. Secondary metabolites of PhIP are formed via acetyltransferase (AT) and sulfotransferase (ST) activity; however, which is responsible for its clastogenic effect is unknown. We addressed this question. We used a parental Chinese hamster lung cell line and three sublines transfected with different AT genes to test the clastogenic (i.e., micronucleus-inducing) effects of metabolically activated PhIP and 7,12-dimethylbenz[a]anthracene (DMBA) in the presence and absence of pentachlorophenol (PCP), a ST inhibitor. PhIP was significantly more clastogenic in the three AT-enriched sublines than in the parental line (p < 0.001). DMBA (a ST-activated mutagen), on the other hand, equally induced MNs in all the cell lines. When PCP was added to the test system, the MN-induction ability of DMBA, but not of PhIP, decreased significantly (p < 0.001). These findings strongly suggest that PhIP clastogenicity is due to AT activity and not to ST activity.