We investigated the ability of IgM-, IgD-, and IgG-bearing cells from the spleens of unprimed (BALB/c x C57BL/Ka)F1 mice to restore the adoptive primary anti-BSA and anti-DNP antibody responses. Purified populations of isotype-specific cells were prepared by immunofluorescent staining and sorting on the fluorescence activated cell sorter. Bright or dull cells were transferred to irradiated syngeneic recipients which were challenged with DNP-BSA in complete Freund's adjuvant. Unfractionated spleen cells as well as IgM- and IgD-bearing cells restored the adoptive primary IgM and IgG antibody response. IgG-bearing cells restored a vigorous adoptive response which was all IgG (2-mercaptoethanol resistant). Depletion of IgG-bearing cells markedly increased the adoptive IgM response, and depletion of IgM-bearing cells markedly increased the IgG response. However, depletion of IgD-bearing cells resulted in a considerable reduction in the IgG response. The latter finding indicates that there is a subpopulation of IgD-bearing cells which express little or no surface IgM and which make a considerable contribution to the adoptive primary IgG response.