A reverse-phase, high-performance liquid chromatography method is described for the quantification of 4'-hydroxymephenytoin formed enzymatically from 14C-labeled (S)-mephenytoin following incubation with cDNA-expressed CYP2C19 or human liver microsomes. Analytical separation is achieved using a C18 column developed with a gradient from 10 to 100% methanol, with detection using a scintillation detector. This method is applicable to enzymatic studies for determination of CYP2C19-catalyzed (S)-mephenytoin 4'-hydroxylation activity.