The gastrocnemius, a fast-twitch white muscle, and the soleus, a slow-twitch red muscle, were studied in A/J mice. The specific activities of the lysosomal hydrolases, JJ-D-glucuronidase, hexosaminidase, f6-D-galactosidase and arylsulphatase, the innermitochondrial-membrane enzyme cytochrome c oxidase, and the outer-mitochondrialmembrane enzyme monoamine oxidase, were greater in the soleus than in the gastrocnemius. The