Coagulation assays in citrated plasma and platelet-aggregation patterns in citrated platelet-rich plasma were performed, using human, baboon, and canine blood. Similar fibrinogen concentrations, factor VIII antihemolytic factor (AHF) clotting protein concentrations, and thrombin times were seen in human and baboon plasma, whereas prothrombin times and activated partial thromboplastin times were significantly (P less than 0.017) more prolonged in baboon plasma than in human plasma. Canine plasma had significantly lower prothrombin times, activated partial thromboplastin times and thrombin times, and significantly higher concentrations of fibrinogen and factor VIII (AHF) clotting protein than did human plasma. The baboon factor VIII antigen cross-reacted with an antibody against human factor VIII antigen, whereas the canine factor VIII antigen did not. Human and canine platelets had similar aggregation patterns to adenosine diphosphate (ADP), whereas baboon platelets were less responsive to ADP than were human platelets. The response to collagen-induced aggregation in human and baboon blood was similar at concentrations of 0.190, 0.100, 0.050 and 0.025 mg/ml, whereas the response in human and canine blood was similar at concentrations of 0.190, 0.100, and 0.050 mg/ml. The lag time before aggregation with collagen was 2 to 3 times longer for canine platelets than for human platelets; human and baboon platelets had similar lag times. Baboon platelets were more responsive to arachidonic acid than were human platelets at concentrations of 0.25, 0.12, and 0.06 mg/ml, whereas canine platelets were less responsive than were human platelets at the highest concentration of 0.5 mg/ml. Human platelets were more responsive to epinephrine than were baboon or canine platelets.(ABSTRACT TRUNCATED AT 250 WORDS)