One of us has previously reported that treatment of the Keilin and Hartree heart-muscle preparation with 2,3-dimercaptopropanol (BAL), in the presence of air, leads to the complete inactivation of the succinate oxidase system with little if any effect on the activities of succinate dehydrogenase (until more than half the BAL was oxidized) or cytochrome c oxidase. The inactivation of the complete succinate oxidase system requires the oxidation of BAL by air in the presence of the enzyme. It is not caused by H2O2 or BAL disulphides produced during the oxidation of BAL. Spectroscopic studies identified the block as lying between cytochromes b and c. It was suggested that a BAL-labile factor is present which transfers electrons from cytochrome b to cytochrome c and which is destroyed by coupled oxidation with BAL. The factor is also required for NADH oxidation. Subsequent work showed it is not identical with cytochrome c1 (ref. 4), myoglobin present in the preparation or the antimycin-binding site. We report here that this factor is identical to the iron-sulphur protein in the central portion of the respiratory chain first identified by Rieske.