We previously showed that radioactive N-ethylmaleimide injected intramuscularly reacts with actomyosin and other muscle proteins and that a transfer of these modified proteins to lysosome-rich large granules was associated with their degradation (Gerard, K. W., and Schneider, D. L., (1979) J. Biol. Chem. 254, 11798-11805). We now show that muscle cells, when challenged by an increase in proteins modified with N-ethylmaleimide, can increase degradation by increasing the activities of enzymes involved in protein turnover. Cathepsin B activity increased 2-fold 36 h after injection of N-ethylmaleimide. In contrast, non-lysosomal proteolytic enzymes, calcium-dependent protease, and leucine aminopeptidase, did not significantly increase. Lysosomes are also involved in the degradation of normal muscle proteins labeled with [3H]leucine. Treatment with chloroquine, a known inhibitor of lysosome function, resulted in an inhibition of protein degradation, in an increase of the muscle protein content, and in the accumulation of radioactive proteins in lysosomal fractions. Chloroquine treatment for 2 days led to a 270% increase in cathepsin B and a 160% increase in lysosomal ATPase activities, but only a 30% increase in neutral proteinase activities. These results indicate a role for lysosomes in regulation of protein turnover in muscle.