The clinical and biochemical findings in a patient with erythropoietic protoporphyria are described. The patient had an extreme accumulation of protoporphyria in his erythrocytes and there was a thirty-fold increase in stool protoporphyrin. The patient also had elevated protoporphyrin in serum, bone marrow and liver. The porphyrins of the red blood cells, bone marrow and serum had fluorescence spectra different from that of the liver. Urine showed increased copra-, hepta- and uroporphyrins. Ferrochelatase activity in the bone marrow was less than 20% of that of non-porphyric controls. In spite of the extremely high levels of red cell protoporphyrin, the patient was clinically healthy with modest changes in liver function tests and liver histology. The results are discussed in relation to the development of liver damage and the recent findings of the effect of porphyrins on cell metabolism.