This paper reviews our recent studies of blood coagulation activation in the chimpanzee which were carried out employing sensitive immunoassays that measure activation markers of blood coagulation in plasma. Infused factor VIIa activated both factors IX and X in vivo; this reaction depended on the formation of the factor VIIa-tissue factor (TF) complex. The infusion of endotoxin also led to assembly of the factor VIIa-TF complex, enhancing fibrin formation. This process occurred through the intermediate action of specific cytokines.