A highly sensitive assay procedure for 5,5-diphenylhydantoin (DPH) involves oxidation with alkaline permanganate to form benzophenone. This is then extracted with heptane, allowed to react with concentrated sulfuric acid, and the fluorescence of the reaction product is measured. The procedure appears to be fairly specific for DPH; 5.(p.hyd roxyphenyl)-5.phenylhydantoin (a DPH metabolite), phenobarbital, or other anticonvulsant drugs do not interfere. The sensitivity of the fluorometric procedure permits assays on finger-blood specimens. It provides a simple means for evaluating the effectiveness of a given dose schedule in different subjects.