Hormonally responsive adenyl cyclase activity was measured in lysates of human platelets. Activity was stimulated by fluoride, prostaglandin E /, and glucagon; and was inhibited by thrombin, epinephrine, norepinephrine, and serotonin. The results suggest that adenosine-3 /, 5 / monophosphate (cyclic AMP) may be important in the regulation of platelet adhesiveness.