A cDNA clone designated as CYP2C43 was isolated from the rhesus monkey liver cDNA library. The first 16 amino acid residues at the N-terminal region of this cDNA product were identical with those of P450 CMLd which have been purified and characterized as S-mephenytoin 4'-hydroxylase in monkey liver. The respective nucleotide and deduced amino acid sequences of CYP2C43 were 83% and 77%, identical to those of monkey CYP2C20. Antibody against CYP2C9 detected a protein in the microsomes of yeast transformed CYP2C43 expression plasmid. The specific content of recombinant CYP2C43 was 78.0 pmol/mg protein and the yield was 4.23 nmol/l of the culture. CYP2C43 was able to metabolize S-mephenytoin stereo-selectively. The activity for S-mephenytoin in the microsomes reconstituted with or without cytochrome b(5) was found to be 96.2 or 23.7 pmol/min/nmol P450, respectively. CYP2C43, however, did not show any oxidative activity for tolbutamide. These results indicate that CYP2C43 is the second identified member of the monkey CYP2C subfamily and a cDNA clone encoding P450 CMLd in monkey.