Decrease in ATP availability in blood platelets occurs during metabolic blockage or storage and is associated with loss of functional response of the cells. This suggests that ATP-consuming processes are involved in execution of function and maintenance of a responsive state for resting cells. Energy consumption by platelet contractile proteins may account for the energy requirement. We intend to study the function related behavior of actin and myosin in resting and stimulated platelets. Actin: A considerable amount of ADP is bound to platelet actin and can be fractionated since it is not ethanol extractable. In resting platelets this ADP has a rapid turnover and probably represents a major ATP consuming process. We will study the kinetics of this turnover in relation to the functional state of platelets. Myosin: Platelet myosin is phosphorylated in intact platelets in response to thrombin-stimulation. We propose to study the parameters of myosin phosphorylation in intact platelets so its role in platelet function might be determined. These studies will attempt to elucidate the molecular mechanisms of the ATP-utilizing processes that are involved in platelet function.