The overall objective of the project is to elucidate the mechanisms whereby heparin and antithrombin III inhibit blood coagulation. Since a number of reactions which lead to coagulation are affected by these materials, the data obtained from this research should clarify many aspects in the regulation of blood clotting. The research will deal with the following subjects: 1.) Heparin will be investigated with respect to the specific functional groups required for its unique activities. This will involve studies on the effects of modifying specific functional groups of heparin on its actions in inhibiting factors Xa and IXa in the presence of antithrombin III. Analogous studies were conducted previously on the inhibition of thrombin and it will be of special interest to ascertain whether the same functional groups in heparin are involved in all the activities. 2.) Studies will be made to elucidate the interactions of heparin with antithrombin and the serine protease clotting factors. The possible preparation of radioactive antithrombin for these studies will be investigated. Among other techniques to be employed will be difference spectroscopy and the effects of active site inhibitors. 3.) The possible effects of mucopolysaccharides of the circulatory system on the regulation of antithrombin will be investigated. 4.) Experiments will be performed to fractionate heparin according to its biological activity.