The nature of signals that govern the development of immunoglobulin heavy chain-dependent B cells is largely unknown. Using mice deficient for the B cell-expressed Src-family protein tyrosine kinases (SFKs) Blk, Fyn and Lyn, we show an essential role of these kinases in pre-B cell receptor (pre-BCR)– mediated NF-κB activation and B cell development. This signaling defect is SFK specific, as a deficiency in Syk, which controls pre-B cell development, does not affect NF-κB induction. Impaired NF-κB induction was overcome by the activation of protein kinase C (PKC)-λ, thus suggesting the involvement of PKC-λ in pre-BCR–mediated SFK-dependent activation of NF-κB. Our data show the existence of a functionally distinct SFK signaling module responsible for pre-BCR–mediated NF-κB activation and B cell development.