1. Liver biopsies from two patients with the Dubin—Johnson—Sprinz syndrome were subjected to analytical subcellular fractionation by sucrose-density-gradient centrifugation and enzymic micro-assays.
2. A selective deficiency of mitochondrial superoxide dismutase has been demonstrated in these patients.
3. The significance of this enzyme deficiency is discussed in relation to the organelle pathology of the syndrome.