Six cytochrome P-450 (P-450) isozymes were purified to electrophoretic homogeneity from the livers of four human organ donors, with three of these isozymes purified from a single individual. Differences were noted between all six P-450s for some or all of the parameters determined by the techniques of sodium dodecyl sulfate-polyacrylamide gel electrophoresis, peptide mapping, spectral analysis of ferrous-carbon monoxide complexes, double-diffusion immunoprecipitin analysis or crossed immunoelectrophoresis (sodium dodecyl sulfate-polyacrylamide gel electrophoresis/peroxidase-coupled staining) with rabbit antisera raised to five of the P-450s, or catalytic activity toward d-benzphetamine, benzo[a]pyrene, acetanilide, debrisoquine, (R)- and (S)-warfarin, and 1-naphthylamine. While NADPH-fortified human liver microsomal preparations showed catalytic activity toward trichloroethylene, 7-ethoxycoumarin, 2-naphthylamine, and 2-aminofluorene in addition to the other substrates mentioned, none of the P-450s which we purified from these microsomes catalyzed the oxidation of these compounds in reconstituted enzyme systems containing purified rat liver NADPH-P-450 reductase. Antibodies raised against one of the purified P-450s inhibited d-benzphetamine N-demethylase activity in microsomal incubations but did not inhibit the metabolism of 7-ethoxycoumarin, acetanilide, benzo[a]pyrene, or debrisoquine. The data provide a strong biochemical basis for the view that distinct isozymes of P-450 exist in humans and that these isozymes differ in catalytic activity toward drugs and carcinogens.