Polyamino acids containing diiodotyrosine, glutamic and or aspartic acids manifest instant anticoagulant activity on intravenous administration. The action of these substances appears to be, at the least in part, due to an inhibition of the late steps in the coagulation pathway. Ca ions, chelation is not involved. Preparation of such materials by thermal condensation of amino acid and by base catalyzed polymerization of their protected N-carboxy anhydride monomers is under study. While the thermal condensation method only permits iodination of the performed polyamino acid, the possibility of synthesizing the iodopeptides from diiodotyrosine N-carboxy anhydride derivatives is being explored. Future work is directed toward the establishment of the molecular properties necessary for maximum anticoagulant activity, i.e. optimum amino acid molar ratios, degree of iodination and molecular weight. Further studies are going to be directed toward the elucidation of the action mechanism on the late steps of the coagulation pathway especially their effect on the polymerization of the fibrin monomer units. Their potential involvement in thrombin inhibition will also be investigated.