The ubiquitously expressed, error-prone DNA polymerase β (polβ) plays a role in base excision repair, and the involvement of this molecule in the nonhomologous end joining (NHEJ) process of DNA repair has recently been demonstrated in yeast. Polβ-deficient mice are not viable, and studies on conditional mutants revealed a competitive disadvantage of polβ^−/− vs. wild-type cells. We show here that polβ-deficient mice survive up to day 18.5 postcoitum, but die perinatally; a circumstance that allowed the investigation of a potential role of polβ in lymphocyte development by transfer of fetal liver cells (FLC) derived from polβ^−/− embryos into lethally irradiated hosts. FLC transfers using mutant cells lead to an almost normal reconstitution of the lymphocyte compartment, indicating that polβ-deficiency does not prevent V(D)J recombination, which is known to employ factors of the NHEJ pathway. Mice reconstituted with polβ^−/− FLC mount a normal T cell-dependent immune response against the hapten (4-hydroxy-3-nitrophenyl) acetyl (NP). Moreover, germinal center B cells from NP-immunized reconstituted mice show normal levels and patterns of somatic point mutations in their rearranged antibody genes, demonstrating that polβ is not critically involved in somatic hypermutation.