T hree siblings with intense jaundice and hemolytic anemia at birth were found to excrete a high level of coproporphyrin in their urine and feces; the pattern of fecal porphyrin excretion was atypical for hereditary coproporphyria because the major porphyrin was harderoporphyrin (greater than 60%; normal value is less than 20%). The lymphocyte coproporphyrinogen III oxidase activity of each patient was 10% of control values, which suggests a homozygous state. Both parents showed only mild abnormalities in porphyrin excretion and lymphocyte coproporphyrinogen III oxidase activity decreased to 50% of normal values, as is expected in heterozygous cases of hereditary coproporphyria. Kinetic parameters of coproporphyrinogen III oxidase from these patients were clearly modified, with a Michaelis constant 15-20-fold higher than normal values when using coproporphyrinogen or harderoporphyrinogen as substrates. Maximal velocity was half the normal value, and we also observed a marked sensitivity to thermal denaturation. The possibility that a mutation affecting the enzyme on the active center which is specifically involved in the second decarboxylation (from harderoporphyrinogen to protoporphyrinogen) was eliminated by experiments on rat liver that showed that coproporphyrinogen and harderoporphyrinogen were metabolized at the same active center. The pattern of porphyrin excretion and the coproporphyrinogen oxidase from the three patients exhibited abnormalities that were different from the abnormalities found in another recently described homozygous case of hereditary coproporphyria. We suggest naming this variant of coproporphyrinogen oxidase defect "harderoporphyria."