S-Warfarin 7-hydroxylation, S-flurbiprofen 4'-hydroxylation, and diclofenac 4'-hydroxylation activities were determined in liver microsomes of 30 humans of which 19 were wild-type (Arg144.Ile359), 8 were heterozygous Cys (Cys144.Ile359), and 3 were heterozygous Leu (Arg144.Leu359) allelic variants of the cytochrome P450 2C9 (CYP2C9) gene. All of the human samples examined contained P450 protein(s) immunoreactive with anti-CYP2C9 antibodies in liver microsomes. Individuals with the Cys144 allele of CYP2C9 had similar, but slightly lower, activities for the oxidations of these substrates than those of wild-type CYP2C9. One of the three human samples heterozygous for the Leu359 allele had very low Vmax and high Km values for the oxidation of three substrates examined, while the other two individuals gave kinetic parameters comparable to those seen in the wild-type and Cys144 CYP2C9. Reverse transcriptase-polymerase chain reaction analysis, however, showed that all of the three human samples with the heterozygous Leu359 variant were found to express both Ile359 and Leu359 variants at relatively similar extents in liver RNA of three humans. These results suggest that the Cys144 variant of CYP2C9 catalyzes the CYP2C9 substrates at rates comparative to, but slightly lower than, those of wild-type CYP2C9, while the Leu359-allelic variant has slower rates for the oxidation of these drug substrates. Activities for the oxidation of these CYP2C9 substrates in humans with heterozygous Leu359 allele is likely to be dependent on the levels of expression of each of the wild- and Leu-variants in the livers. However, one of the humans with a heterozygous Leu allele was found to have very low activities towards the oxidation of CYP2C9 substrates. The basis of this defect in catalytic functions towards CYP2C9 substrates is unknown.