Liposomes containing membrane IL-1, Iak, and the antigen conalbumin were evaluated as "synthetic antigen presenting cells." The role of these three molecules in macrophage-T cell interaction was studied by testing their ability to induce the proliferation of a T-cell clone specific to conalbumin (the D10 cell line) or immune spleen cells sensitized three times in vivo with conalbumin. In the latter case, splenic macrophages were eliminated by adherence and a lysomotropic agent. The antigen conalbumin was presented on the surface of the liposomes as native undigested protein. When the liposomes presented native conalbumin, Iak, and membrane IL-1, significant proliferation occurred, but if the liposomes lacked membrane IL-1, the proliferation of the T-cell clone and the spleen cells reached only about 60 percent of the previous signal. Native conalbumin and class II antigen alone were required for T-cell activation, while membrane IL-1 only amplified the response. When the liposomes were made with only Iak and membrane IL-1, lacking conalbumin, there was no proliferation of antigen-specific target cells. These results indicated that in this synthetic system, membrane IL-1 increases the magnitude of the response but is not essential for the proliferative response of antigen-specific T cells.