B ymphocyte stimulator (BLyS) is a member of the tumor necrosis factor (TNF) superfamily. BLyS stimulates proliferation of, and immunoglobulin production by, B cells. However, the relative importance of BLyS in physiological B cell activation is unclear. We identified a B cell receptor for BLyS through expression cloning as TACI, an orphan TNF receptor homologue of unknown function. Binding of BLyS to TACI activated signaling by nuclear factor-κB (NF-κB). In vitro soluble TACI-Fc fusion protein blocked BLyS-induced NF-κB activation in B lymphoma cells and IgM production in peripheral blood B cells. In vivo treatment of immunized mice with TACI-Fc inhibited production of antigen-specific IgM and IgG1 antibodies and abolished splenic germinal center (GC) formation. Thus, BLyS activity must play a critical role in the humoral immune response.