Annexin V, a protein with a high affinity and a strict specificity for aminophospholipids at physiologic calcium concentrations, was used to probe platelet activation and the development of procoagulant activity. Platelet secretion was studied in parallel using VH10, a murine monoclonal antibody specific for GMP-140, an alpha-granule membrane glycoprotein. Both proteins were labeled with fluorescein isothiocyanate and platelet activation was assessed by flow cytometry. Microparticles, which are shed from the platelet surface and also support procoagulant activity, were distinguished from platelets according to their associated light scattering signal. The relative ability of different inducers to trigger exposure of the procoagulant surface and microparticle formation was: ionophore A23187 = thrombin plus collagen = collagen = thrombin. The density of aminophospholipid on microparticles was higher than on remnant platelets. Platelet activation by these agonists was accompanied by GMP-140 exposure, both on platelets and microparticles. Here, thrombin was the most efficient agonist. The mechanisms responsible for the above processes were investigated using E-64-d, a specific membrane-permeable inhibitor of Ca(2+)-activated protease (calpain); tetracaine, an activator of calpain; and N-ethylmaleimide and diamide, two sulfhydryl-reactive agents. These agents were added to platelets alone or before stimulation by agonists. Calpain activity was assessed by the hydrolysis of cytoskeletal proteins as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Results showed that calpain activity is not essential for aminophospholipid translocation or for secretion. In contrast, although sulfhydryl-reactive agents alone can trigger procoagulant activity, they inhibit microvesicle formation and platelet secretion induced by the above agonists, suggesting that different mechanisms account for these phenomena. The use of annexin V in flow cytometry is a rapid method to assess procoagulant activity in platelets and the loss of phospholipid asymmetry in cell membranes.