Abstract
1. Effects of the platelet secretagogues thrombin and Concanavalin A (Con A) on shape change and release have been studied under conditions where external Ca2+ and ADP have been controlled. 2. In the absence of detectable aggregation both Con A- and thrombin-induced release of serotonin are markedly dependent upon extracellular calcium. 3. Con A, unlike thrombin, does not produce aggregation in calcium-supplemented medium. 4. The above property of Con A suggests that, in combination with added creatine phosphate and creatine phosphokinase, this ligand will be of value in the analysis of calcium-dependent release in platelets.