Specific hapten-binding B cells were identified in the splenic marginal zones following immunization with hapten-protein conjugates. Hapten binding by marginal zone B cells does not appear to be due to passive absorption of anti-hapten antibody. For double immunization with two haptens, 2,4-dinitrophenyl (DNP) and 2-phenyloxazalone (Ox) each conjugated to hemocyanin, resulting in the appearance of discrete DNP-binding cells and Ox-binding cells in the marginal zone. Very few cells were identified which bound both haptens. The hapten-binding cells in the marginal zones have a phenotype characteristic of other marginal zone B cells. They express surface IgM but not IgD. Occasional cells also have surface IgG2c. All hapten-binding cells possessed the antigen recognized by the monoclonal antibody HIS 14 but lacked those identified by HIS24 and HIS22. Hapten-binding B cells were shown to have been in cell cycle shortly before entering the marginal zone but were no longer in cell cycle after arriving at that site. Once in the marginal zone hapten-binding cells were shown to remain in that site for upwards of 2 weeks. Following reimmunization with DNP-hemocyanin, DNP-binding but not Ox-binding cells were lost from the marginal zone. At the same time DNP-binding cells arrived in the periarteriolar lymphocytic sheath and to a lesser extent the follicles. These cells were in active cycle and appeared to give rise both to plasma cells and marginal zone hapten-binding cells. It is concluded that hapten-binding cells found in the marginal zones are memory B cells i.e. they have been derived from B cells which have undergone antigen-driven proliferation, they are no longer in cell cycle but can be induced to re-enter cell cycle by subsequent exposure to antigen. Good antibody responses were obtained following immunization with hapten-polysaccharides; however, no hapten-binding cells appeared in the marginal zones in response to these T cell-independent type 2 antigens.