The oxidative metabolism of antipyrine, hexobarbital, coumarin, zoxazolamine, 7-ethoxycoumarin, and the chemical carcinogen benzo[a]pyrene (BP) was studied in 32 adult human livers obtained at autopsy. When enzyme activity for one substrate was plotted against enzyme activity for a second substrate for each of the 32 livers, statistically significant correlations were found between the rates of metabolism of BP and the rates of metabolism of each of the other five drug substrates. The degree of correlation was dependent upon the substrate pair that was studied. Highly significant statistical correlations (p less than 0.001) for monooxygenase activities among the different livers were observed for BP with antipyrine (r = 0.85), antipyrine with zoxazolamine (r = 0.82), antipyrine with hexobarbital (r = 0.79), zoxazolamine with 7-ethoxycoumarin (r = 0.75), antipyrine with coumarin ( r= 0.72), zoxazolamine with coumarin (r = 0.72), BP with hexobarbital (r = 0.72), hexobarbital with coumarin (r = 0.71), BP with zoxazolamine (r = 0.69), hexobarbital with zoxazolamine (r = 0.64), coumarin with 7-ethoxycoumarin (r = 0.61), and BP with coumarin (r = 0.57). Less significant correlations were obtained for BP with 7-ethoxycoumarin (r = 0.35; p = 0.05). It is not known whether the relationships between the metabolism of the several substrates described here for autopsy livers would also occur with fresh livers. The lack of a perfect correlation for any of the substrate pairs suggests the presence in human liver of multiple monooxygenase enzyme systems for the metabolism of benzo[a]pyrene and the five other substrates studied, as well as heterogeneity in their distribution among the 32 livers that were examined. The approach described in the present report may have significance in the study of the comparative metabolism of drugs, chemical carcinogens, and other environmental pollutants by human tissues and may help us find predictor drugs that will be useful for evaluating the drug- and carcinogen-metabolizing capacity of different individuals in the human population.