To shed light on the nature of abnormal porphyrin metabolism in Dubin-Johnson syndrome (DJS), coproporphyrin isomer distribution in urine and bile was investigated in carriers and patients with DJS selected from among the families in the cluster area of the syndrome in Japan. Urinary total coproporphyrin content in patients with DJS exceeded that of normal controls (P less than 0.01). However, the increase in urinary excretion of total coproporphyrin was of a much lesser degree compared to a remarkable increase in urinary total coproporphyrin reported for erythropoietic porphyria in which a deficiency of uroporphyrinogen III cosynthetase had been demonstrated. Urinary total coproporphyrin content in carriers was significantly smaller than that of normal controls (P less than 0.05). This paradoxical decrease of urinary total coproporphyrin concentration in DJS carriers was observed in consequence of a decrease in coproporphyrin III excretion without concomitant increase in type I isomer. Administration of griseofulvin, in the dose which had been shown to precipitate attacks of acute porphyria in susceptible subjects, to carriers and patients with DJS did not elicit clinical signs suggestive of porphyria, and urinary total coproporphyrin concentration and the percentage of coproporphyrin I remained unchanged. These results were interpreted to demonstrate an aspect of DJS that was not consistent with the hypothesis that a partial defect in uroporphyrinogen III cosynthetase of the liver might be the cause of DJS. Coproporphyrin in the bile of DJS patients was preponderantly type I.