Male and female neonates, children and adults, and patients with hemolytic anemia, in whom subnormal erythrocyte glucose-6-phosphate dehydrogenase deficiency was demonstrated on spectrophotometric assay, were all detected using the methylene blue reduction screening test. This finding and the results of mixing experiments suggested that it is as sensitive as the assay. Offering advantages over other screening tests, it is useful for surveys and for routine laboratory work.