Release of membrane vesicles from human erythrocytes was induced by modulation of red cell ATP levels, by incubation of erythrocytes with sonicated dimyristoylphosphatidylcholine (DMPC) suspensions, or by a sequential combination of both procedures. When red blood cell ATP levels were decreased prior to incubation with DMPC, the lag-time between addition of the lipid and beginning of vesiculation was reduced. Furthermore, the rate of vesicle release itself was accelerated. Experiments carried out with a rapid ATP depletion technique showed that the onset of vesiculation and the release were most evidently accelerated in those cases where echinocytes had been formed prior to the addition of DMPC. The results suggest that red blood cells with reduced cellular ATP levels or an altered cell shape are more susceptible to a further perturbation of the membrane by addition of exogenous DMPC.