We determined whether primed and unprimed B cells in the spleen of (BALB/c × C57BL/Ka)F(1) mice contain subpopulations that express a predominant surface Ig isotype. Spleen cells were stained for surface isotypes and sorted on the fluorescence-activated cell sorter (FACS) in order to obtain B cells bearing predominantly IgM (μp cells), IgD (δp cells), or IgG (γp cells). Each population was assayed for its capacity to restore the adoptive primary and secondary anti-bovine serum albumin (BSA) antibody response in irradiated syngeneic recipients. In addition, the adoptive response restored by isotype-predominant cells was compared to that restored by isotype- positive cells (B cells bearing a given surface isotype alone or in combination with others). The experimental results show that μp cells restore the adoptive primary and secondary IgM and IgG responses to BSA, and γP cells restore only the primary and secondary IgG response. Δp Cells restored the adoptive secondary IgG response, but failed to restore the adoptive primary response at the cell doses tested. ΓP Cells but not δp cells suppressed the IgM response of the μ(+) and δ(+) cells. The contribution of isotype-predominant cells to both the adoptive primary and secondary anti-BSA response was smaller than that of B cells bearing a combination of surface isotypes. Differences in the Ig isotype pattern expressed on the surface of primed and unprimed B cells are discussed.