The cytochromes P-450 metabolize a wide array of compounds, including xenobiotics such as drugs and carcinogens, and endogenous compounds such as steroids. The focus of this project is the characterization of structure-function relationships and regulation of the multiple forms of this enzyme. Monoclonal antibodies (MAbs) to rat P-450s are a tool in these studies. A 3- methylcholanthrene (MC)-inducible P-450 was immunopurified from the livers and lungs of rats. On the basis of amino acid sequence analysis, peptide mapping, and molecular weight, the liver and lung P-450s were indistinguishable. Using a MAb to ethanol-inducible rat liver P-450, a P-450 has been purified from both rat and human liver. These differed in primary structure as evidenced by different amino terminal sequences and peptide maps. Developmental regulation of P-450 was examined by studying P-450-dependent testosterone metabolism in 3- and 24- month old rats. Ring hydroxylation patterns, as well as content of liver constitutive P-450s, varied with age. While P-450 activities generally declined with age, the 7 alpha-hydroxylase and corresponding P-450 form responsible for this activity increased with age.