The interaction of human peripheral blood lymphocytes with liposomes containing DNP-aminocaproylphosphatidylethanolamine together with either egg yolk or dipalmitoylphosphatidylcholine has been investigated. When lymphocytes were incubated with liposomes at 37°C, the aqueous compartment (86Rb+) and the lipid portion (3H-lipid) of the liposomes became cell associated to an equivalent extent. At 0°C, however, the incorporation of 3H-lipid exceeded that of 86Rb+. Lymphocyte-liposome interactions were accompanied by the transfer of DNP to the surface of the lymphoid cell as measured by susceptibility to complement in the presence of anti-DNP antibody. Hapten transfer was not limited to liposome interactions with lymphocytes, but occurred also with other cells (e.g., Chang cells). Hapten transfer could also be demonstrated by susceptiblity to K cell-mediated lysis. These findings suggest that liposomes may prove to be a useful vehicle for the transfer of new antigenic determinants onto cell surfaces. The implications of these findings are discussed in the context of using liposomes as targets for cell-mediated cytotoxic attack.