Platelets in a dilute platelet-rich plasma clot formed by the addition of thrombin at 4 C do not lose their alpha-granules or develop platelet-fibrin interconnections. These clots do not retract or lyse. If after clot formation for 30 minutes the clots are transferred to 37 C, the alpha-granules then disappear from the platelet, extensive platelet-fibrin interconnections develop, and the clots retract and lyse. When the dilute plasma clots, however, are formed at 37 C, alpha-granules are quickly lost, few platelet-fibrin interconnections develop, and the clots do not retract or lyse. Deoxyglucose and antimycin, when added before thrombin at 4 C, prevent the loss of alpha-granules and development of platelet-fibrin interconnections after transfer of the clot to 37 C. These inhibited platelets did not retract or lyse. From the results reported here we conclude that the development of platelet-fibrin interconnections as well as the retraction and lysis of the clot occur in conjunction with the release of alpha-granules from the platelet. It may be necessary for this release to occur after the fibrin network is completed, since release of the alpha-granules in the 37 C clot before clot formation did not result in formation of many platelet-fibrin interconnections or in clot retraction or lysis.