Smad2 is a member of the intracellular mediators that transduce signals from TGF-β receptors and activin receptors. Targeted inactivation of Smad2 in mice leads to early lethality before gastrulation. It was shown previously that TGF-βRII deficiency in vivo leads to defects in B cell homeostasis, Ag responsiveness, and IgA class switch recombination of B cells. To investigate the importance of Smad2-mediated signaling in B lymphocytes, we generated a B cell-specific inactivation of Smad2 in mice (bSmad2-/-). bSmad2 -/- mice had normal B cell numbers in the spleen but showed a reduced population of marginal zone B cells. In contrast, B cells in Peyer's patches and peritoneal B-1a cells of bSmad2-/- mice were increased in numbers. bSmad2-/- mice showed a reduced number of surface-IgA + B cells and of IgA-secreting cells in Peyer's patches, decreased levels of IgA in serum, and, after immunization with a T cell-dependent Ag, a reduced IgA response. Class switch recombination to IgA was impaired in Smad2-deficient B cells, when stimulated in vitro with LPS in the presence of TGF-β. The growth-inhibitory effects of TGF-β in LPS-stimulated B cells were not affected in Smad2-deficient B cells. In summary, our data indicate a crucial role of Smad2 in mediating signals for the TGF-β-directed class switch to IgA and the induction of IgA responses in vivo. Other B cell functions like growth-inhibitory signaling, which are known to be regulated by signals via the TGF-βR, are not affected in Smad2-deficient B cells.