We have utilized several methods to isolate populations of murine splenic B lymphocytes bearing different classes of surface Ig molecules. Antibody and complement-mediated cytotoxicity was used to remove IgD-bearing B cells, and fluorescein-conjugated anti-IgM and polyspecific anti-Ig-F(ab′)2 antibody fragments were used with a fluorescence-activated cell sorter to isolate cells bearing IgM or all Ig classes. First, the technique of isolating cells in the sorter was evaluated. It was found that among cells sorted as negative for Ig there was always a certain percentage where IgM reappeared on the membrane after ovenight culture. Second, evidence is presented that surface IgM is present on and controls the proliferation of the majority of B lymphocytes in an adult mouse spleen. This includes cells bearing predominantly IgM and cells bearing predominantly IgD. Finally, a theory of B cell stimulation is presented, based on distinctive roles for surface IgM and IgD molecules, as well as a balance between their signals to an individual cell.