The mechanisms of membrane fusion and fission are important because many of the dynamic features of the behaviour of biological membranes, including exocytosis, depend on such reactions (Ohki et al., 1988), and benefits to medical science may therefore result from an improved understanding of these mechanisms at the molecular level. This paper is concerned with the participation of osmotically driven events in the fusion of erythrocytes, and their possible relevance to other fusion-fission reactions.