Microsomes were prepared from human livers obtained from renal donors of various ages and both sexes. Their drug-metabolizing capacity was measured and compared to that of rat liver microsomes. The following parameters were investigated: cytochrome P-450, cytochrome B5, NADPH-cytochrome c reductase, epoxide hydrolase, aryl hydrocarbon hydroxylase, benzphetamine N-demethylase, p-nitroanisole-O-demethylase, ethoxycoumarin-O-deethylase, steroid-16 alpha-hydroxylase. In addition, the metabolism of benzo(a)pyrene, progesterone, pregnenolone, testosterone, dehydroepiandrosterone and estradiol was studied in detail in vitro. The inhibitory effect of metyrapone and alpha-naphthoflavone on 7-ethoxycoumarin-O-deethylase was measured. The microsomal proteins of both species were separated by polyacrylamide gel electrophoresis in the presence of dodecyl sulfate. The following conclusions were drawn from the results obtained. Human liver microsomes can be stored under optimal conditions for the measurement of a large variety of enzymic activities. Human liver microsomes are able to metabolize the various xenobiotics used as substrates with a rate similar to that of female rat liver microsomes. No sex-linked difference in enzymic activity was observed in human microsomes. Significant differences in benzo(a)pyrene and steroid metabolism were registered when human and rat liver microsomes were compared. The monooxygenase activities, the sensitivity to in vitro alpha-naphthoflavone and metyrapone, the results of steroid metabolism, and slab gel electrophoresis are strong indications for multiplicity of human liver cytochrome P-450.