A chicken antibody mono-specific to cytochrome P450C-M/F, which exists in untreated male and female rat liver and catalyses the 2- and 16 alpha-hydroxylation of estrogens (1), was used to screen a cDNA library of male Sprague-Dawley rat liver. Four cDNA clones which encoded P450 isozymes, CMF1a, CMF1b, CMF2 and CMF3, were isolated. CMF1a and CMF2 deduced consisted of 504 and 500 amino acid residues, respectively, while C-terminal 487 and 324 residues for CMF1b and CMF3, respectively, were deduced from the 5'-truncated cDNAs. The isozymes were more than 72% similar in amino acid sequences to each other and to rat P450db1, P450db2 (2), and to a mouse male specific C-P45016 alpha (3), suggesting that they belonged to a new P450 subfamily, P450IID. CMF1a and db1, and CMF2 and db2, respectively, were 99.2% and 99.0% similar in amino acid sequences, suggesting that they were virtually identical. CMF1a and CMF1b were different but 96.1% similar, and CMF3 was between 76% and 78% similar to other members of the rat P450IID family.