Benzo[a]pyrene (B[a]P)-tetrols formed after stereoselective cytochrome P450-dependent metabolism from (-)-trans-7,8-dihydroxy-7,8- dihydrobenzo[a]pyrene [(-)-B[a]P-7, 8-diol] by lung microsomes (n = 19) and peripheral blood lymphocytes (n = 13) from lung cancer patients were measured, and the effect of smoking explored. B[a]P-tetrols were quantified by an HPLC/fluorescence assay with a detection limit of approximately 300 attomol, after incubation with peripheral blood lymphocytes or microsomes from lung cancer patients who were current cigarette smokers, ex-smokers and non-smokers. In lymphocytes from these subjects, high, medium and low metabolic activities respectively for (-)-B[a]P-7,8-diol to tetrol conversion were found, but there was no statistically significant difference between smokers, ex-smokers and non-smokers. When the B[a]P-tetrol formation by human lung microsomes was measured, recent smokers had 4- to 7-fold higher (P = 0.04) metabolic activity than ex-smokers and non-smokers. The mean lung microsomal arylhydrocarbon hydroxylase (AHH) activity was three times higher in smokers than in non-smokers and was undetectable in ex-smokers. AHH activity was correlated with tetrol formation in the same lung microsomal samples (r = 0.62, P less than 0.01 in smokers; and r = 0.67, P less than 0.01 in all subjects). When subjects were grouped according to smoking habits, however, no correlation was detected between mean tetrol formation by lung microsomes and that of lymphocytes. Thus, lymphocytes cannot serve as a surrogate for lung microsomes concerning the pulmonary metabolism of (-)-B[a]P-7,8-diol. The much higher B[a]P-tetrol formation observed in lung microsomes from smokers is in accord with a reported higher pulmonary AHH activity, cytochrome P450IA level, and CYP1A1 gene expression in recent tobacco smokers.