TH 2.52, a subline of the B cell hybridoma with Iab, Iad, and IgM molecules on the cell membrane, was treated with F(ab')2 fragments of purified goat anti-mouse mu antibody (anti-mu), and the change in the expression of surface Ia molecules was determined by microcytotoxicity assays, quantitative absorption tests, and analyses of flow microfluorometery (FMF). We have previously reported that TH 2.52 cells can markedly generate IgM after stimulation with anti-mu without T cell factors. In the present studies, it was shown that the expression of surface Iab molecules on TH 2.52, originated from normal B cells of C57BL/6 (B6) mice, significantly decreased after treatment with anti-mu. In contrast, Iad molecules derived from M12.4.1 lymphomas did not change under the same conditions. These results indicate that cross-linking of anti-mu with surface IgM molecules on TH 2.52 provides signals for differentiation into IgM-secreting cells; this is followed by a decrease in the expression of Iab molecules on the cell membrane. Furthermore, monoclonal anti-Ia antibody (anti-Ia) did not inhibit the generation of IgM-secreting cells by TH 2.52 cells treated with anti-mu. In addition, la- sublines of TH 2.52 obtained after mutagenesis with ethyl methanesulfonate (EMS), as well as the original TH2.52, could differentiate into IgM-secreting cells in the presence of anti-mu. These findings suggest very strongly that la molecules on the cell membrane are not required for the induction of IgM secretion by B cells treated with anti-mu.