Several toxins interfere in blood coagulation and platelet aggregation. We study the structure-function relationships and mechanism of anticoagulant and antiplatelet proteins, particularly PLA2 enzymes. The strongly anticoagulant PLA2 binds to factor Xa and specifically inhibits the prothrombinase complex. Based on the anticoagulant site of PLA2, we are attempting to identify the functional sites of coagulation factors. We also determine the structure of venom prothrombin activators that are structural and functional homologs of coagulation factors. Based on their structures, we are designing anticoagulant peptides.