Sequencing of genomic polymerase chain reaction (PCR) products synthesized using primers generated from the CYP2C8 and CYP2C9 cDNAs revealed the presence of a new CYP2C gene in the human genome. Primers specific to exons of this new gene were used to perform PCR on human liver cDNA libraries and cDNA synthesized from human liver mRNA to generate a cDNA containing a complete cytochrome P450 amino acid reading frame. This cytochrome P450 cDNA, designated CYP2C18, displayed 85% and 87% nucleotide and 77% and 81% amino acid sequence similarities, respectively, with cDNAs and proteins corresponding to CYP2C8 and CYP2C9. cDNA-directed synthesis of CYP2C18 revealed a protein with relative Mr 49,000 on sodium dodecyl sulfate-polyacrylamide gels, which is considerably less than that calculated from the deduced amino acid composition, Mr 55,747. A preferred substrate for this enzyme has not been uncovered. Levels of CYP2C8, CYP2C9, and CYP2C18 mRNAs were examined in 17 human liver specimens using a PCR-based assay. CYP2C18 mRNA was found in all livers examined, albeit at mean levels 7-8-fold lower than those of mRNAs encoding CYP2C8 and CYP2C9. Marked interindividual differences in levels of expression of all three CYP2C mRNAs were also found.