The in vitro effect of several bacterial endotoxins on human platelets was determined. Nine different endotoxins failed to induce aggregation in platelet-rich plasma (PRP) or of platelets washed by two different methods; four of them which we studied further failed to induce [14C]serotonin release in PRP. In contrast, using recently described test systems for platelet coagulant activity, all the endotoxins shortened the latent period occurring before aggregation of a mixture of washed platelets, normal serum, and CaCl2, and the clotting time of this mixture upon addition of fibrinogen. Washed platelets obtained from PRP preincubated with endotoxin had a higher platelet coagulant activity than platelets obtained from PRP preincubated with buffer. Washed platelets contribute to thrombin generation by providing factor V, a factor X activator and possibly phospholipid. Since the endotoxins did not influence the factor V activity of platelets or the platelet factor 3 activity, either in PRP or using platelets washed by albumin density gradient centrifugation, it is suggested that they enhance the factor-X activator activity of human platelets.