A genetic polymorphism causing deficient metabolism of the anticonvulsant drug mephenytoin occurs in 5% of the Caucasian and 23% of the Japanese population. By monitoring the activities of the two major oxidative pathways of mephenytoin metabolism in the column eluates, we have purified from human livers a cytochrome P-450 isozyme, P-450 meph, which exclusively and stereoselectively catalyzes the 4-hydroxylation of (S)-mephenytoin, the major pathway affected by the polymorphism, whereas P-450 meph was virtually devoid of catalytic activity for N-demethylation of mephenytoin, the pathway remaining unaffected by the genetic deficiency. P-450 meph had an apparent Mr of 55 000 and a lambda max in the reduced CO-binding spectrum of 450 nm. Polyclonal rabbit antibodies against purified human P-450 meph almost completely inhibited the 4-hydroxylation of mephenytoin but had little effect on N-demethylation in human liver microsomes. In microsomes of liver biopsies of two subjects characterized in vivo as 'poor metabolizers' of mephenytoin, immunocrossreactive and immunoinhibitable material was observed with similar or identical properties to those of P-450 meph. There was no difference in the extent of the immunochemical reaction between microsomes of in vivo phenotyped poor metabolizers and extensive metabolizers of mephenytoin. These data suggest that P-450 meph is the target of the genetic deficiency and support the concept that a functionally altered variant form of P-450 meph causes this polymorphism.