The transcription factors EBF1 and Pax5 have been linked to activation of the B cell lineage program and irreversible loss of alternative lineage potential (commitment), respectively. Here we conditionally deleted Ebf1 in committed pro-B cells after transfer into alymphoid mice. We found that those cells converted into innate lymphoid cells (ILCs) and T cells with variable-diversity-joining (VDJ) rearrangements of loci encoding both B cell and T cell antigen receptors. As intermediates in lineage conversion, Ebf1-deficient CD19(+) cells expressing Pax5 and transcriptional regulators of the ILC and T cell fates were detectable. In particular, genes encoding the transcription factors Id2 and TCF-1 were bound and repressed by EBF1. Thus, both EBF1 and Pax5 are required for B lineage commitment by repressing distinct and common determinants of alternative cell fates.