Introduction During the fusion of the membrane of secretory vesicles with the plasma membrane, the outer leaflet of the plasma membrane becomes continuous with the inner leaflet of the vesicle membrane and the cytosolic leaflets of the two membranes become continuous. As both membranes have an asymmetric distribution of phospholipids between the two leaflets of the bilayer, any description of the rearrangements of phospholipids during the fusion process must take this topographic feature into consideration. In erythrocytes, the phospholipids of the plasma membrane are asymmetrically distributed, with the choline phospholipids concentrated in the outer leaflet and the aminophospholipids concentrated in the inner leaflet [ 11. This arrangement is maintained by an aminophospholipid translocase which catalyses the ATP-dependent translocation of phosphatidylserine (PS) and phosphatidylethanolamine (PE) from the extracellular leaflet to the cytosolic leaflet of the membrane (reviewed in [I]). Clearly the asymmetric lipid distribution is an important determinant of fusion: erythrocytes in which lipid asymmetry is abolished are fused with one another more easily [2, 31 and with virus [4], perhaps because the external leaflet of these cells is enriched in aminophospholipids. Remarkably the secretory vesicle membrane harbours the same translocase activity [5, 61, also oriented towards the cytosolic leaflet, and the lipids which form the vesicle are asymmetrically distributed [7]. Hence