Human IL-1, recombinant murine IL-1 and E. coli LPS were found to be potent inducers of plasminogen activator (PA)-inhibitor activity, both in vivo, in rats, as well as in cultured human endothelial cells. In vivo, LPS rapidly and dose-dependently (0.01-1,000 micrograms/kg) increased plasma PA-inhibitor activity. Infusion of IL-1 into rats resulted in a small but significant increase in PA-inhibitor activity in rat plasma. Likewise, in cultured human umbilical vein endothelial cells, LPS and IL-1 induced increased synthesis of PA-inhibitor. We suggest that the induced rat plasma inhibitor might be of endothelial origin.