Three inhibitory monoclonal antibodies specific to cytochrome P450 3A4/5 (CYP3A4/5), CYP2C8/9/19 and CYP2E1, respectively, were used to assess the contribution of the P450s to the metabolism of seven substrates in liver microsomes from 18 human donors, as measured by monoclonal antibody inhibition phenotyping of the substrate conversion to product(s). Metabolism of seven substrates by recombinant cytochromes P450 and human liver microsomes was performed in the presence of monoclonal antibodies and their metabolites were analyzed by high-performance liquid chromatography (HPLC) or gas chromatography-mass spectrophotometry (GC-MS) to measure the magnitude of inhibition. Our results showed that CYP3A4/5 contributes to testosterone 6beta-hydroxylation, taxol phenol formation, diazepam 3-hydroxylation, diazepam N-demethylation, and aflatoxin B1 3-hydroxylation in human liver by 79.2%, 81.5%, 73. 2%, 34.5% and 80%, respectively. CYP2E1 contributes to chlorzoxazone 6-hydroxylation, p-nitroanisole O-demethylation, and toluene hydroxylation by 45.8%, 27.7% and 44.2% respectively, and CYP2C8/9/19 contribute to diazepam N-demethylation by 30.6%. The additive contribution (75.3%) of human CYP3A and CYP2C to diazepam N-demethylation was also observed in the presence of both anti-CYP3A4/5 and anti-CYP2C8/9/19 monoclonal antibodies. The contribution of individual P450s to the specific metabolic reaction in human liver varies greatly in the individual donors and the substrates examined. Thus, inhibitory monoclonal antibodies could play a unique role in defining the single or subfamily of cytochrome P450 that is responsible for the metabolism of specific drugs.