A highly purified preparation of mouse B cells showed greatly decreased incorporation of [6-3H]-thymidine when stimulated with pokeweed Pa-1 mitogen or bacterial lipopolysaccharide compared with mouse splenic lymphocytes. This decreased stimulation was restored by the addition of purified T cells, but not macrophages. Nylon-adherent T cells exerted this helper activity only toward complement receptor-positive B cells (CR+B cells). whereas the helper activity of nylon-non-adherent T cells was effective only on complement receptor-negative B cells (CR-B cells). Since the helper activity of nylon-adherent T cells was completely abolished by the treatment with anti-Ia antiserum and complement but that of nylon-non-adherent T cells was not, it was assumed that Ia+T cells were helper cells for CR+B cells and Ia-T cells helper cells for CR-B cells. Moreover, these helper activities of both T-cell subsets were mediated by soluble factors, which were effective just before the onset of DNA synthesis of the corresponding B-cell subpopulations.