The purpose of the studies included in this proposal are to determine the effect of physical forces on elements of the blood. We have developed a technique for measuring elastic and viscous moduli of clots. We have shown that the elastic modulus particularly is related to the fibrinogen concentration and to the concentration of platelets in plasma as it clots. We have also been able to demonstrate that factor XIII is important in determining the elastic modulus of clots. This technique appears to have value in defining problems in clotting and we intend to expand these studies to study a variety of coagulation abnormalities. In studies of the effect of shear stress on platelets we have been able to demonstrate that platelets are exquisitely sensitive to injury and destruction by shearing stress. Our previous studies have been done in platelet-rich plasma; we intend to expand these studies to demonstrate the effect of shear stress on platelets in whole blood. We have also been able to demonstrate that shear stress results in abnormalities of thrombin-fibrinogen reactivity. These studies are to be expanded to determine the effects of such stresses on fibrinogen using a thrombo-kinetic assay.