The three phospholipase A2 isoenzymes from Naja nigricollis crawshawii snake venom inhibit both blood coagulation and platelet aggregation. To investigate the correlation between phospholipid splitting ability of these enzymes and their inhibitory activities, the effects of various preincubation times and the inclusion of EDTA were examined. Preincubation of plasma and thromboplastin with the phospholipase isoenzymes resulted in an increase in Ca2+-initiated clotting time with time of preincubation. Incubation of the isoenzymes with EDTA before their addition to the plasma-thromboplastin mixture reduced the anticoagulant effect. These results indicate that the catalytic activity contributes at least partially to the anticoagulant effect. However, inhibition of platelet aggregation appears to be independent of enzymatic activity since there is no increase in inhibition with time of preincubation of platelets and phospholipases, and inclusion of EDTA has no significant effect on inhibition of aggregation. All three enzymes appear to belong to class B of the platelet affector PLA2 enzymes as determined by platelet effects, since they do not initiate platelet aggregation.