A mouse monoclonal antibody, MBS 105, raised against rabbit liver microsome cytochrome P-448 (I), had no effect on the activity of the benzo[a]pyrene hydroxylase system from rabbit liver microsomes.The antibody was covalently bound to CH-activated Sepharose and incubated with rabbit cholate-solubilized liver microsomal fractions.A protein of mol. weight 55,000 was adsorbed, but could not be subsequently eluted from the gel.A reconstituted system comprising immobilized cytochrome P-448, NADPH-cytochrome P-450 reductase, and dilauryl-α-lecithin was capable of oxidizing benzo[a]pyrene.