Abstract Mitochondrial fractions were isolated from skeletal muscle of control (C57 BL 6J dy/+) and dystrophic (C57 BL 6J dy/dy) mice, and enzymatic activities (cytochrome c oxidase, rotenone-insensitive NADH cytochrome c reductase) were determined. After electrophoretic separation, calcium-binding proteins were identified. An important anomaly was observed in the mitochondria of dystrophic muscle, i.e., a considerable reduction of a specific calcium-binding protein (61,000 Da mol. wt.).