Phosphoinositide-3 kinase (PI3K) is thought to activate the tyrosine kinase Btk. However, through analysis of PI3K^−/− and Btk^−/− mice, B cell antigen receptor (BCR)-induced activation of Btk in mouse B cells was found to be unaffected by PI3K inhibitors or by a lack of PI3K. Consistent with this observation, PI3K^−/− Btk^−/− double-deficient mice had more severe defects than either single-mutant mouse. NF-κB activation along with Bcl-x_L and cyclin D2 induction were severely blocked in both PI3K^−/− and Btk^−/− single-deficient B cells. Transgenic expression of Bcl-x_L restored the development and BCR-induced proliferation of B cells in PI3K^−/− mice. Our results indicate that PI3K and Btk have unique roles in proximal BCR signaling and that they have a common target further downstream in the activation of NF-κB.