IB- is a nuclear IB protein robustly induced in macrophages and fibroblasts upon TLR or IL-1R stimulation. IB- associates with NF-B in the cell nucleus and is essential for the induction of a subset of secondary response genes represented by IL-6. Here, we analyzed induction of IB- in mouse B cells and found that IB- is induced by BCR or TLR stimulation. Similar to TLR stimulation, BCR stimulation elicited NF-B-mediated transcriptional activation and mRNA stabilization of IB- via a cis-element in IB- mRNA. Proteasome inhibitors inhibited transcriptional activation but not post-transcriptional activation, indicating independency of the two signals. Co-stimulation of the BCR and TLR9 or TLR7, but not TLR2/1, synergistically induced IB-. Co-engagement of inhibitory Fc receptor suppressed BCR-mediated IB- expression but not that induced by TLR stimulation alone or co-stimulation of TLR and the BCR. The PI3K inhibitor LY294002 inhibited BCR-mediated, but not TLR-mediated, induction of IB-, consistent with the role of PI3K in BCR signaling and its suppression by FcR. Analysis of IB--deficient B cells demonstrated that IB- was essential upon stimulation of BCR or TLR for the expression of several genes including IL-10 and CTLA4. IB--deficient B cells exhibited impaired proliferation and enhanced up-regulation of CD86 following stimulation of TLR9, but not the BCR, indicating critical roles for IB- in TLR signaling in B cells. Strict regulatory mechanisms for the induction of IB- via multiple pathways and its essential function upon stimulation indicate that IB- plays an important role in B cells.