To clarify the mechanisms underlying the loss of body protein during fever and sepsis, we incubated rat muscles with highly purified human leukocytic pyrogen. This polypeptide, which appears identical to interleukin-1, is released by leukocytes and signals the onset of fever in the hypothalamus. In muscles incubated at 37 degrees C, leukocytic pyrogen stimulated net protein degradation by 62 to 118 per cent (P less than 0.001). Proteolysis increased, but rates of muscle-protein synthesis did not change. The pyrogen also dramatically stimulated muscle synthesis of prostaglandin E2, which promotes protein breakdown in this tissue. Addition of indomethacin with leukocytic pyrogen prevented prostaglandin E2 synthesis and abolished the increase in proteolysis. The acceleration of protein breakdown induced by pyrogen was also blocked by Ep-475, an inhibitor of lysosomal thiol proteases. When muscles were incubated at 39 degrees C to mimic fever, protein breakdown increased, but addition of leukocytic pyrogen caused a further marked increase in proteolysis and prostaglandin E2 production. Thus, human leukocytic pyrogen can act on skeletal muscle to stimulate intralysosomal proteolysis by increasing the production of prostaglandin E2. These findings suggest that cyclooxygenase inhibitors may be useful in the treatment of negative nitrogen balance in fever. In addition, the release of prostaglandin E2 induced by leukocytic pyrogen may account for the myalgia that accompanies fever.