Progressive exercise intolerance was associated with a decreased maximal rate of ubiquinol cytochrome c reductase (complex III) activity in the muscle mitochondria of the studied patient and with a thirty five-fold increase in the I50 for antimycin A. In contrast, myxothiazol sensitivity was not altered. Complex III activity was stable at 37 degrees C, but progressively decreased at 4 degrees C. An heteroplasmic G to A mutation at position 15615 of the mitochondrial DNA, resulting in the replacement of the highly conserved Gly290 in cytochrome b by Asp, was identified. Histochemical studies showed increased cytochrome oxidase and succinate dehydrogenase activities under the sarcolemma of type I fibres. After partial extraction of mitochondria from the muscle, the residual pellet contained a lower percentage of the mutation than did whole muscle, suggesting that the percentage of mutation is higher in the most readily extracted mitochondria, most probably present under the sarcolemma. In the current 8 transmembrane helix model of cytochrome b, Gly290 lies at the end of the sixth transmembrane helix, facing the intermembrane space and close to the presumed sites of interaction between cytochrome b, the iron-sulfur protein and the 9.5 kDa protein. Since immunoblotting experiments showed a relative decrease in the proportions of these three subunits in the patient's mitochondria compared with the other complex III subunits, it is probable that the complex III instability and the relative decrease in these subunits are related to the mutation. The relationship between the decrease in the apparent affinity for antimycin A and the instability of complex III are discussed.