The cytokine interleukin-1 (IL-1) is involved in a wide range of inflammatory and immune responses. As such, IL-1 could play a role in peripheral nerve repair mechanisms. Specifically, by its already established properties as a regulator of nerve growth factor (NGF) synthesis, and as a chemotactant to macrophages. We examined, therefore, IL-1 production in injured mouse peripheral nerve. Injured nerve segments were incubated in serum free medium to produce conditioned medium (CM) that was then tested for IL-1 activity in a thymocyte proliferation assay. CM induced thymocyte proliferation in a dose-dependent manner. Proliferation was inhibited by the M20 IL-1 inhibitor, the IL-1 receptor antagonist, and antisera raised against recombinant mouse IL-1 alpha. Inhibitions produced by these three specific inhibitors of IL-1-induced thymocyte proliferation strongly suggest that proliferation induced by CM was mediated largely by IL-1 secreted by non-neuronal cells residing in the damaged nerve. IL-1 activity was detected within hours after lesion, and 1 week thereafter. The rapid and prolonged production of IL-1 indicates that IL-1-dependent mechanisms can play roles in the response of the peripheral nerve to injury: degeneration and regeneration. The regulation of NGF synthesis, and the recruitment of white blood cells, macrophages in particular, from blood into the damaged nerve tissue, are two such mechanisms.