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The secretion of clotting factors by rat spleen macrophages and human peripheral blood monocytes has been studied. The results show that the amount of clotting factors measured depends critically upon the characteristics of the assay system used. The presence of warfarin, salicylic acid or thrombin in the culture medium is shown to decrease the vitamin K dependent clotting factor activity in the supernatant after in vitro culture of rat spleen macrophages and human peripheral blood monocytes.

Endothelial cells isolated from rat aorta were infected in vitro with rat cytomegalovirus. Viral antigens appeared in nucleus and cytoplasma and newly made extracellular virus was detected in the supernatant. Furthermore, the viral infection caused the appearance of procoagulant activity on the endothelial cells.

It is shown that Echis carinatus venom activates both coagulation factor II and coagulation factor X. The activation of both proenzymes by the venom is Ca++-dependent; phospholipids are not necessary. The activation of factor II by the venom is a slow process and, in the absence of factor X, the clotting activity towards fibrinogen is generated only very slowly. Because Echis carinatus venom clots plasma readily, we postulate that under conditions where the prothrombinase complex can be formed from the factor X activated by the venom it is this complex, rather than the venom itself, that is responsible for the major part of the thrombin formation.