Although it has been known for some time that organophosphate chemicals containing the P = S moiety are irreversible inhibitors of cytochrome P450, this knowledge has not been generally applied to the human metabolism of xenobiotics. Recent studies have demonstrated that organophosphate insecticides containing this moiety are potent inhibitors of the metabolism of both xenobiotics and endogenous substrates by human liver microsomes and by specific human cylochrome P450 isoforms.