In early work done more than ten years ago in this laboratory, observations were made which indicated a role for osmotic swelling in chemically induced cell fusion (Ahkong et al., 1973~). At that time it was suggested that: “As we have never observed fusion between the ovalappearing, unswollen erythrocytes, and as every chemical found to induce cell fusion in this laboratory also causes cells to become spherical, we conclude that cell swelling by colloidal osmosis plays an essential role in cell fusion” (Ahkong et al., 19736). More recently, Finkelstein and his colleagues have shown that phospholipid vesicles can be induced to fuse with a planar, phospholipid bilayer by osmotic forces. They have suggested that osmotic swelling of intracellular vesicles may be essential for the fusion of biological membranes in exocytosis (Akabas et al., 1984), and this is consistent with findings that solutions of high osmolarity can inhibit the exocytosis of chromaffin granules in adrenal medullary cells (Hampton & Holz, 1983; Pollard et al., 1984) and the exocytosis of cortical granules in sea-urchin eggs (Zimmerberg & Whitaker, 1985).