Increasing evidence has demonstrated the importance of monocyte procoagulant activity (PCA) in the pathogenesis of coagulopathies in a variety of diseases. Because endotoxin precipitated coagulopathies are common sequelae to intestinal ischemia/endotoxemia in the equine species, we investigated the ability of equine peripheral blood monocytes to express PCA. Monocytes isolated from five healthy adult horses were incubated in vitro with Escherichia coli endotoxin (10 micrograms), and the PCA was measured by the ability of cellular lysates to accelerate the clotting times of equine plasma in a modified one-stage recalcification assay. Equine monocyte PCA was identified as thromboplastin based on lack of clot formation in factor VII-deficient plasma. The induction of PCA occurred as early as 2 hr after endotoxin exposure, peaked at 6 hr (396% increase), and then gradually declined. The amount of PCA was proportional to the dose of endotoxin (0.01 to 100 micrograms) and the number of monocytes. Neither platelets nor neutrophils produced PCA, either in the absence or presence of endotoxin (1 microgram). Lymphocytes at a concentration of 4 x 10(6)/ml RPMI did produce a significant amount of PCA, compared to the time-matched controls. Co-incubation of neutrophils or lymphocytes with monocytes did not alter the PCA, whereas coincubation of platelets and monocytes significantly enhanced the expression of PCA. This effect was further augmented by the addition of endotoxin (1 microgram).