Four monoclonal antibodies (MAbs) to phenobarbital-induced cytochrome P-450 (PB-P-450) show different patterns of inhibition of PB-P-450 catalyzed aryl hydrocarbon hydroxylase (AHH), 7-ethoxycoumarin deethylase, benzphetamine demethylase and ethylmorphine demethylase. The inhibition constants vary depending on the individual monoclonal antibody and the individual substrate. Two of the four monoclonal antibodies completely inhibit the reduction of cytochrome P-450 by NADPH cytochrome c (P-450) reductase. The same cytochrome P-450 bound to carbon monoxide, however, can be reduced chemically by sodium dithionite in the presence of the monoclonal antibody. These data indicate that the two MAbs examined completely prevent electron transfer by NADPH cytochrome c (P-450) reductase. Substrate binding is partially inhibited by the monoclonal antibody. The type I substrate-binding spectrum of benzphetamine is inhibited more than the type II binding spectrum of aniline. The degree of inhibition of the substrate binding as indicated by the spectrum is less than that observed for the inhibition of catalytic enzyme activity by the monoclonal antibodies. The data indicate that each of the MAbs are directed toward epitopes on the cytochromes P-450 with different relationships to the active catalytic site.