An assay procedure for dihydropteridine reductase in peripheral leukocytes is described. The assay utilizes the tetrahydropterin-dependent reduction of ferri-cytochrome C in the presence of NADH and requires a smaller number of cells than assays described for cultured skin fibroblasts. Dihydropteridine reductase activity was not detectable in the peripheral leukocytes nor in the cultured skin fibroblasts from two adolescent patients with malignant hyperphenylalaninemia. The parents of the patients showed approximately 50% of normal dihydropteridine reductase activity in their peripheral leukocytes. Immunochemical experiments using antibodies against bovine liver dihydropteridine reductase suggest that normal leukocytes and skin fibroblasts contain dihydropteridine reductase which is immunologically similar to that of human liver. The present studies indicate that the determination of dihydropteridine reductase activity in peripheral leukocytes can be used to diagnose hyperphenylalaninemia due to a defect in dihydropteridine reductase.