Phosphatidylcholine liposomes bound to the surface of L cells inhibit cell attachment to L-cell monolayers or to lipid films. Aggregation of L cells or of mouse embryo fibroblasts is also diminished upon treatment with liposomes. However, they neither inhibit cell attachment to glass or cellulose acetate substrata, nor diminish conA-mediated cell aggregation. It is supposed that liposome-binding sites on the cell surface described earlier are involved in cell-cell attachment.