Coumarin 7-hydroxylation and other cytochrome P-450-associated enzyme activities were studied in human liver biopsy homogenates and compared with activities in livers of other species. Coumarin 7-hydroxylation is extraordinarily active in human liver biopsy samples in vitro. Activity is lower in mouse, rabbit or guinea pig liver and essentially absent in rat liver. Cytochrome P-450 content and other associated enzyme activities were higher in animals. Coumarin 7-hydroxylation is induced by phenobarbitone in mouse liver, but no significant increase was seen in human or rat liver after exposure to inducers. Correlations amongst coumarin 7-hydroxylase, aryl hydrocarbon hydroxylase, 7-ethoxycoumarin O-deethylase and cytochrome P-450 are statistically significant (r values from 0.56 to 0.73), but do not permit the conclusion, that the same P-450 form catalyzes all the reactions studied. The correlations between coumarin hydroxylation and antipyrine half-life or clearance are statistically significant, but not good enough for predictive purposes. Coumarin 7-hydroxylase in human liver is inhibited by alpha-naphthoflavone, SKF 525A, metyrapone and aniline.