The structural similarity of related forms of P450 makes selective immunoinhibition of individual forms notoriously difficult to achieve. We report the use of a targeted antibody to overcome this problem. An antibody was raised against the synthetic peptide, Ser-Lys-Lys-Gly-Pro-Arg-Ala-Ser-Gly-Asn-Leu-Ile, corresponding to residues 291-302 of human CYP1A2. This sequence of human CYP1A2 is located in a similar position to a proinhibitory region previously identified in rat CYP1A1 and CYP1A2. The antibody bound strongly and specifically to CYP1A2 in human hepatic microsomal fraction. Binding was unaffected by denaturation of the protein. The specificity of the antibody was demonstrated by immunoblotting of human hepatic microsomal fraction where a single immunoreactive band was identified at Mr 54,000. The intensity of this band correlated strongly with high-affinity phenacetin O-deethylase activity of the microsomal fractions. In addition, the antibody bound to a single protein at Mr 54,000 in the microsomal fraction of lymphoblastoid cells expressing human CYP1A2, but not to any other recombinant P450 enzyme. CYP1A2-dependent activity (high-affinity phenacetin O-deethylase) of human hepatic microsomal fraction was inhibited >90% by whole antiserum or purified immunoglobulin. This decrease in activity represents complete inhibition of CYP1A2 activity, residual phenacetin O-deethylase activity being due to low-affinity enzymes. In contrast, the antibody, which does not bind to rat CYP1A2, had no effect on CYP1A2-dependent activity (high-affinity phenacetin O-deethylase) of rat hepatic microsomal fraction. The antiserum also had no effect on human hepatic microsomal debrisoquine 4-hydroxylase (CYP2D6) or coumarin 7-hydroxylase (CYP2A6) activities, indicating that inhibition was specific to human CYP1A2. These results demonstrate the importance of the region comprising residues 291-302 of human CYP1A2 in the catalytic activity of this enzyme.