Germfree BALB/c mice have been treated from birth with intraperitoneal injections of purified goat antibodies to mouse IgM. The treated mice, and controls which had received an equivalent amount of goat gamma-globulin, were sacrificed at 8 or 13 wk of age. Compared to controls, mice given anti-micro (a) had very few germinal centers in spleen and lymph node, (b) had decreased numbers of mature plasma cells synthesizing IgM and IgG1 in spleen, and virtual absence of IgA-synthesizing plasma cells in the gut, (c) had greatly diminished numbers of B lymphocytes bearing membrane-bound immunoglobulins of the IgM, IgG1, IgG2, and IgA classes in spleen, (d) had reduced synthesis of IgM, IgG2, and IgA by in vitro spleen cultures, and (e) had significant decreases in serum levels of IgM, IgG1, IgG2, and IgA. The treated animals failed to make antibodies to ferritin after hyperimmunization, and lacked natural antibodies to sheep erythrocytes. These results indicate that cells ultimately committed to synthesis of IgG1, IgG2, and IgA immunoglobulins are derived from cells which have expressed IgM determinants at an earlier stage of differentiation. They are consistent with a proposed two-stage model for plasma cell differentiation. The first stage is antigen independent, involves sequential activation of Cmicro, Cgamma, and Calpha genes by progeny of a single stem cell, and results in the formation of B lymphocytes bearing membrane-bound recognition antibodies of each class. The second, antigen-dependent, stage results in formation of mature plasmacytes and memory cells.