Structural perturbations of positively charged sonicated liposomes (L) induced by incubation with human erythrocytes have been studied by thin-section electron microscopy. It has been found that just after mixing with cells L bind to cell surfaces and fuse with each other, first forming larger L and then flattened multilamellar structures. Successive fusion events occurring in the latter result in a reduced number of bilayers which gradually comes to a single one.