We have investigated whether or not tissue factor (TF) which is present in the supernatant of isolated glomeruli, is responsible for the stimulatory activity of TXB2 production by isolated human platelets. Reconstituted TF stimulated TXB2 synthesis in platelets in a dose-dependent manner. This effect was potentiated in the presence of a mixture of the major fatty acids found in glomerular supernatants. Addition of a neutralizing anti-TF monoclonal antibody abolished both the procoagulant activity and the platelet-TXB2 stimulatory activity of reconstituted TF and of glomerular supernatants. Anti-factor VII/VIIa (F VII/VIIa) Fab inhibited in a dose-dependent manner the platelet-TXB2 stimulatory activity of an identical dilution of reconstituted TF and of glomerular supernatants, providing evidence that the functional complex TF. VIIa and not TF itself was the active agent. Pretreatment of platelets, TF or glomerular supernatant by hirudin, an inhibitor of thrombin, as well as by antithrombin III heparin, which inhibits both activated factor X and thrombin also markedly inhibited the synthesis of TXB2 by platelets in the presence of either TF or glomerular supernatant. Taken together, these results demonstrate that the stimulatory activity for TXB2 production by platelets which is released by the glomerular cells is attributable to TF. TF does not act directly. Its effect is mediated by thrombin which is formed de novo at the platelet surface in the presence of even traces of the plasma coagulation proteins associated with platelets. TXB2 formation in platelets correlates well with TF concentration in the glomerular supernatant. The possibility of a similar set of mechanisms associated with glomerular injury may require consideration.