Recent studies have claimed that interleukin 1-containing preparations increase skeletal protein degradation similar to that seen during infection and inflammation. However, preparations employed have contained other products of activated macrophages, including tumor necrosis factor-alpha. In the present report, we investigated the capability of recombinant-derived murine and human interleukins 1-alpha and 1-beta and human tumor necrosis factor-alpha to affect skeletal protein synthesis and degradation both in vitro and in vivo. Partially purified products of Staphylococcus albus-stimulated human blood monocytes increased skeletal protein degradation both in vivo and in vitro. However, none of the recombinant interleukin 1 nor the human tumor necrosis factor-alpha preparations had any impact on skeletal protein balance. Both recombinant interleukin 1 and tumor necrosis factor-alpha stimulated the production of prostaglandin E2 (PGE2). Furthermore, a polyclonal antibody to human interleukin 1 eliminated the lymphoproliferative response to partially purified monocyte preparations (interleukin 1 activity), but failed to abrogate the increased skeletal protein degradation in vitro. This study demonstrates that although interleukin 1 and tumor necrosis factor-alpha induce a PGE2 response by skeletal muscle in vitro, some macrophage product distinct from either interleukin 1 or tumor necrosis factor-alpha is responsible for the accelerated skeletal protein degradation seen with partially purified human blood monocyte products. Elevated PGE2 levels do not appear to regulate skeletal protein balance in vitro.