The role of endogenous natural IgM in promoting the adaptive Ab response was investigated in newly constructed mutant mice in which B cells do not secrete IgM but still express surface IgM and IgD and undergo class switching to express other Ig isotypes. While the mutant mice had relatively normal numbers of conventional B (B-2) cells in all tissues examined, unexpectedly, B-1 cells in the peritoneum and spleen were approximately threefold more abundant. The elevated levels of B-1 cells were already detectable at 4 wk of age and were stably maintained throughout life. The levels of serum IgG2a, IgG3, and IgA were also elevated in the mutant mice at an early age. IgG2a response to a T cell-independent Ag was augmented, whereas IgG Ab responses to suboptimal doses of a T cell-dependent Ag were impaired. The latter defect was associated with fewer splenic germinal centers, impaired Ab affinity maturation, and less Ag trapping on follicular dendritic cells. Together, these findings demonstrate a physiologic role of natural IgM in the feedback regulation of B-1 cell development, the regulation of IgG2a production, and the promotion of efficient B-2 cell Ab responses.