1. Human erythrocytes were treated with Ca2+ and ionophore A23187 and measurements were made of K+ efflux, polyphosphoinositide breakdown, 1,2-diacylglycerol accumulation, phosphatidate synthesis, changes in membrane polypeptide pattern and release of microvesicles. 2. It was shown that neither transamidase-mediated protein cross-linking, proteolysis of polypeptides 2.1 (ankyrin) or 4.1, nor accumulation of diacylglycerol or phosphatidate appeared to be necessary for microvesiculation to occur. 3. Microvesicles were released only under conditions where KCl efflux leading to cell shrinkage occurred and where polyphosphoinositides were broken down. These circumstances were sufficient to cause microvesiculation only in the presence of increased intracellular concentrations of Ca2+.