The activity of coproporphyrinogen oxidase (E.C. 1.3.3.3) in cultured skin fibroblasts from three patients with hereditary coproporphyria (H.C.) was approximately half that in fibroblasts from normal subjects and patients with other types of porphyria. It is suggested that this is the primary defect in H.C., which is inherited as an autosomal dominant, and that the same abnormality is present in the liver. Consideration of the probable relative activities of the enzymes of haem biosynthesis in the liver in H.C. suggests that the acute attacks of porphyria which are its major clinical manifestation occur when the activity of uroporphyrinogen-I-synthase (E.C. 4.3.1.8) becomes rate-limiting for haem synthesis.