Multilamellar, "fluid" liposomes containing an appropriate concentration of a synthetic dinitrophenylated lipid hapten behave as T-independent (TI), adherent cell-dependent antigens for primary, in vitro, stimulation of hapten-sepcific IgM plaque-forming cells (PFC). The appropriate concentration of hapten is approximately l mol percent with respect to the other liposomal lipids, cholesterol and dimyristoylphosphatidylcholine (DMPC). At a higher hapten concentration, 4 mol percent, liposomes will suppress the anti-dinitrophenyl PFC response to cocultured immunogenic liposomes, but not the anti-sheep cell response to co-cultured sheep cells. This suppression is also TI. The ability of "fluid," haptenated liposomes specifically to stimulate or to suppress the TI PFC response is determined by the ratio of hapten to other lipids in a manner which is independent of the total hapten concentration in culture. This suggests that macrophages, which are required for the observed stimulation of PFC, may not be acting as "antigen presenters" in this system: direct interactions between B cells and liposomes may be effective. The proposed work is concerned with elucidating the role of macrophages in this system and determining the simplest modes of membrane-associated antigen presentation required for specific B cell stimulation and specific B cell tolerance.