A monoclonal antibody (MAb) to a rat acetone-inducible and nitrosamine-metabolizing form of microsomal cytochrome P-450, P-450ac, detected a related P-450 in human liver microsomes by both immunoblot and competitive radioimmunoassay. This MAb was also used to immunopurify microsomal cytochromes P-450 from both human liver and acetone-treated rats; these were electrophoretically homogeneous with apparent molecular weights of 56,200 and 53,000 daltons, respectively. The structures of the cytochromes P-450 were compared by peptide mapping and amino-terminal sequence analyses. They differed in their peptide maps but displayed amino-terminal sequence similarity in their first 19 residues. This report thus demonstrates the utility of MAbs to rat cytochromes P-450 for detection, identification and structural characterization of human P-450s.