A localized burn injury to a rat hindlimb was used to investigate the proteolytic enzymes responsible for the burn-induced increase in muscle protein breakdown. In 10,000 x g pellets of muscle homogenates, burn stimulated (50% to 100%) protease activities with pH optima of 3, 5.5, and 7.8. Burn also stimulated acid protease activity in 27,000 x g supernatants derived from triton X100 treated extracts of muscle. Pretreatment of rats with compound 48/80 (to eliminate contaminating mast cells) eliminated 95% of the neutral protease activity in the particulate fraction. The Ca++-sensitive neutral protease was not affected by either burn or 48/80 treatment. However, muscle extracts from the burned leg always showed a 40% to 70% increase in acid protease activity. All of the acid protease activity could be inhibited by a combination of cathepsin inhibitors pepstatin (0.01 microgram/mL) and leupeptin (1 mumol/L) and leupeptin (1 mumol/L) or Ep475 (1 microgram/mL). Leupeptin and the lysosomotropic agent leucine methyl ester also inhibited the burn-induced proteolysis in intact muscle. A time course shows parallel increases in whole muscle proteolysis and acid protease activity of muscle homogenates. These findings support the conclusion that the increase in lysosomal cathepsins are sufficient to account for the burn-induced increase in protein breakdown in muscles from the injured leg.