Clinical and laboratory findings in 8 patients with erythropoietic protoporphyria are reported. Studies are presented showing that photosensitivity of the phototoxic type can be regularly induced in guinea pigs with the protoporphyrin DC molecule. Erythropoietic protoporphyria in man is probably based on an inborn error of metabolism. All cutaneous abnormalities reported to date as occurring in this disease can be attributed to a photosensitivity mechanism.