Abnormal in vitro red cell photohemolysis can be used as a diagnostic aid for erythropoietic protoporphyria. Photohemolysis is regularly present in erythropoietic protoporphyria and is negligible in normal subjects, in patients with polymorphic light eruption, and in lead poisoning. A simple technique consists of irradiating a suspension of red cells diluted 1:500 in Ringer's solution by using a modified x-ray view box equipped with blacklight fluorescent tubes to serve as the irradiation apparatus. After three hours of exposure presence of photohemolysis is determined by direct visual or by photometric examination of the supernatant fluid. The technique does not involve a chemical or fluorometric analysis.