Interleukin-1 (IL-1) plays a major role in inflammatory responses. Activation of coagulation and fibrin deposition typical of these reactions is mediated by macrophage procoagulants induced on stimulated macrophages. IL-1 activity in the supernatant of lipopolysaccharide (LPS)-stimulated guinea-pig macrophages was markedly enhanced by the presence of thrombin during macrophage activation. Although thrombin alone had no effect, inclusion of 1 mU/ml of thrombin with suboptimal levels of LPS produced a 200-fold increase in IL-1 activity, and further enhancement was observed with increasing doses of thrombin. The active site of thrombin was necessary for enhancement, as the serine esterase inhibitor di-isopropyl-fluorophosphate (DIP) and hirudin inhibited the synergy observed with LPS and thrombin. Prothrombin and Factor Xa also enhanced IL-1 production, although not to the same extent as thrombin. Factor Xa-like activity was demonstrated on the surface of LPS-stimulated macrophages. Both the Xa-like activity and IL-1 generated by LPS-stimulated cells were inhibited by heparin. Heparin with a high affinity for antithrombin III (anti-coagulant heparin; HAH) inhibited IL-1 generation, whereas low-affinity heparin (non-anticoagulant; LAH) had no effect. We show that proteases of the extrinsic coagulation cascade enhance IL-1 generation and propose that a Factor Xa-like activity present in activated macrophages, together with thrombin, may be important in IL-1 processing.