By genetically ablating I kappa B kinase (IKK)-mediated activation of the transcription factor NF-kappa B in the B cell lineage and by analyzing a mouse mutant in which immunoglobulin lambda-chain-positive B cells are generated in the absence of rearrangements in the locus encoding immunoglobulin kappa-chain, we define here two distinct, consecutive phases of early B cell development that differ in their dependence on IKK-mediated NF-kappa B signaling. During the first phase, in which NF-kappa B signaling is dispensable, predominantly kappa-chain-positive B cells are generated, which undergo efficient receptor editing. In the second phase, predominantly lambda-chain-positive B cells are generated whose development is ontogenetically timed to occur after rearrangements of the locus encoding kappa-chain. This second phase of development is dependent on NF-kappa B signals, which can be substituted by transgenic expression of the prosurvival factor Bcl-2.