c-Myc, a member of the Myc family of transcription factors, is involved in numerous biological functions including the regulation of cell proliferation, differentiation, and apoptosis in various cell types. Of all of its functions, the role of c-Myc in cell differentiation is one of the least understood. We addressed the role of c-Myc in B lymphocyte differentiation. We found that c-Myc is essential from early stages of B lymphocyte differentiation in vivo and regulates this process by providing B cell identity via direct transcriptional regulation of the ebf-1 gene. Our data show that c-Myc influences early B lymphocyte differentiation by promoting activation of B cell identity genes, thus linking this transcription factor to the EBF-1/Pax-5 pathway.