Incubation of sickle (HbS) erythrocytes for periods up to 96 h leads to the formation of irreversibly sickled cells (ISCs) and to the release of spectrin-free microvesicles similar to those derived from aged or Ca2+-ionophore-treated normal erythrocytes. The sickle microvesicles were somewhat larger than those from normal cells and showed minor differences in their membrane polypeptide composition. Sickle microvesicles were no different from their parent cells in their content of fetal haemoglobin. Neither microvesiculation nor formation of irreversibly sickled cells required the presence of Ca2+ in the medium but Ca2+ did accelerate both processes. Although in these prolonged incubations microvesiculation appeared to occur concomitantly with the formation of ISCs, it is not clear whether or not microvesiculation is a necessary prelude to irreversible sickling.