Alveolar macrophages are intimately involved with fibrin during the course of acute and chronic inflammatory processes in the lung. In this study, the capability of macrophages to impede fibrinolysis was investigated. Human alveolar macrophages obtained by lavage from normal volunteers released a fibrinolytic inhibitor during the first 24 h of in vitro culture but only inconsistently and in some cases (7 of 15) not at all. Lysates of freshly lavaged cells had no activity. Endotoxin, 5 to 100 ng/ml, consistently induced intracellular accumulation and extracellular release of a fibrinolytic inhibitor by cultured macrophages. Induction required protein synthesis. The intracellular and secreted forms of the inhibitor were true plasminogen activator (PA) inhibitors, as judged by their ability to block urokinase-mediated conversion of 125I-plasminogen. On average, 10(7) endotoxin-stimulated macrophages secreted sufficient PA inhibitor during a 24-h culture in vitro to neutralize 2 picomoles urokinase (16 international units). Analysis of the interaction of the human macrophage PA inhibitor with 125I-urokinase (apparent size, 33 kilodaltons) by SDS-gel electrophoresis showed that the enzyme and inhibitor mostly dissociated in SDS, but a stable complex occurred at 60 to 65 kilodaltons and a broad band of enzyme or enzyme-inhibitor complexes between 33 and 40 kilodaltons. Either heat treatment of the inhibitor or active site inhibition of urokinase with p-nitrophenylguanidinobenzoate blocked both types of interaction. The pattern of interaction was virtually indistinguishable from that of a partially purified human placental urokinase inhibitor but different from that of serum protease inhibitors.(ABSTRACT TRUNCATED AT 250 WORDS)