Direct immunofluorescence was used to determine the classes of immunoglobulins expressed on the surface membrane and in the cytoplasm of newborn and adult B lymphocytes differentiating in response to LPS in vitro. In both newborn and adult spleen, a small proportion of IgM-bearing B lymphocytes also stained for IgG2; adult spleen contained an additional population of lymphocytes bearing IgG2 alone. Combined surface and cytoplasmic staining at intervals after culture initiation and demonstrated both IgM and IgG2 on the membranes of the earliest cells synthesizing cytoplasmic IgG2. At later stages the proportion of IgM-IgG2 surface doubles and of cells synthesizing cytoplasmic IgG2 which had surface IgM fell significantly. Detection of surface IgM on IgG2 precursors correlated with susceptibility of the IgG2 precursors to anti-mu-suppression over the first 3 days in cultures of newborn spleen cells. After 3 days when these cells no longer expressed surface IgM, 2gG2 responses were not suppressed although the IgM response was still inhibited. These results suggest that virgin IgG2 precursors may be B lymphocytes expressing both IgM and IgG2, and that "switching" involves the loss of IgM receptors as these cells proliferate and mature.