In addition to the microvesicles released during the treatment of human erythrocytes with Ca2+ and ionophore A23187, a new subpopulation of still smaller dense vesicles (‘nanovesicles’) has now been identified. Nanovesicles are about 60 nm in diameter, have an acetylcholinesterase activity higher than that of microvesicles and appear to be relatively enriched in sphingomyelin and correspondingly depleted of phosphatidylethanolamine. They have a polypeptide composition quite different from those of erythrocyte membranes or microvesicles, consisting largely of components of mol.wts. 60 000 and 26 000 in addition to haemoglobin. These two major polypeptides do not appear to represent contaminating cytoplasmic proteins or proteolytic subfragments of a larger protein.