A new type of screening procedure for the detection of enzymatic defects of the red cell has been described. The blood or red cell sample is added to the reaction mixture. After a suitable period of incubation a drop of the mixture is spotted on filter paper, permitted to dry, and examined for fluorescence under UV light. In this way the oxidation of reduction of pyridine nucleotides is readily evaluated. Reaction mixtures for the detection of glucose-6-phosphate dehydrogenase deficiency, pyruvate kinase deficiency, and glutathione reductase deficiency are described. The same general procedure should be readily adaptable to the detection of other enzymatic deficiencies of red cells, such as phosphogluconate dehydrogenase deficiency or triosephosphate isomerase deficiency.