This study examines the role of neutrophils (PMN) in the pathogenesis of endotoxin-induced microclot formation. It is intended to clarify whether granulocytes are involved in endotoxin-induced activation of intravascular coagulation (generation of soluble fibrin) and/or in endotoxin-induced precipitation of soluble fibrin. Precipitation of soluble fibrin was achieved by injection of endotoxin into ancrod- infused rabbits with circulating soluble fibrin (first model). Activation of intravascular coagulation was elicited by two intravenous injections of endotoxin into rabbits (second model). Seventy-two and ninety-six hours after injection of nitrogen mustard, leukopenic rabbits had PMN counts between 0 and 50 cells per mul. Neutropenia did not prevent the occurrence of glomerular microclots after infusion of ancrod and injection of endotoxin (first model). Neutropenia influenced neither the decrease in mean fibrinogen concentrations nor the drop in mean platelet counts after ancrod and endotoxin administration. In contrast to the first model, neutropenia prevented the occurrence of glomerular microclots and of circulating soluble fibrin after two injections of endotoxin (second model). It did not, however, protect rabbits from the decrease in mean platelet counts after endotoxin administration. These data indicate that granulocytes are involved in endotoxin-induced activation of intravascular coagulation and the production of soluble fibrin but are not essential to endotoxin-induced precipitation of soluble fibrin.