The metabolism of benzo(a)pyrene by human liver microsomes and human lymphocytes has been analyzed by high-pressure liquid chromatography. Human liver forms seven known metabolites and at least five additional unidentified metabolites that migrate as distinct peaks. Lymphocytes incubated with benzo(a)pyrene for 30 min do not form dihydrodiols. Lymphocytes incubated for 24 hr with benzo(a)pyrene form all of the metabolites produced by liver including dihydrodiols as well as additional metabolites. The ratios of phenols formed by liver and lymphocytes are different, and preparations from humans form a different profile of metabolites than that formed by rat liver.