A number of species respond to bacterial endotoxin (lipopolysaccharide [LPS]) wherein cells of the monocyte-macrophage lineage are rapidly induced either directly or via T-cell collaboration to initiate the extrinsic coagulation protease pathway. This results in fibrin formation and deposition as well as consumption of plasma coagulation proteins. It has been claimed that this cellular response, basic to the Shwartzman reaction, is lacking in rats and may account for the more limited severity of the Shwartzman reaction in this species. We examined the in vitro lymphoid procoagulant response in Fischer 344, Brown Norway, and Lewis rats. When peripheral blood mononuclear cells (PBM) were stimulated in vitro with LPS, a procoagulant activity (PCA) response was observed when assayed by acceleration of clotting of recalcified human or rat platelet-poor plasma. The response was rapid, with a maximum achieved at 4 h. PCA was not physically dissociated from viable PBM by 5 mM EDTA, which is consistent with the presence of an intrinsic plasma membrane initiator molecule rather than calcium-bound gamma-carboxylated glutamic acid-containing proteases. The induction of monocyte PCA was prevented by incubation of cells with cycloheximide or actinomycin D, implicating a new biosynthetic requirement. Cultivation of PBM with warfarin did not diminish the function of the effector PCA, nor did vitamin K augment the function of the endotoxin-induced PCA, indicating that the functional activity was not attributable to gamma-carboxylated glutamic acid-containing proteins. No inhibition of the cellular PCA molecule was produced by serine protease inhibitors. The LPS-induced PCA appeared to involve a tissue factor-like molecule since both factors X and VII were required in mediating PCA. Isolation of monocytes and T lymphocytes from LPS-stimulated PBM demonstrated that PCA was present in the monocyte-rich fraction. When isolated rat T lymphocytes and monocytes were separately exposed to LPS, PCA was not induced. In contrast, when the cells were combined, LPS induced PCA, indicating that the PCA response involved cellular collaboration between cells present in T lymphocyte and monocyte populations.