Recently developed B6.kappa(-)lambda(SEG) mice (by crossing kappa(-) and C57BL/6 mice congenic for the wild Mus spretus SEG strain lambda locus lacking genes coding for lambda1 and lambda3) have a very reduced light chain diversity. B6.kappa(-)lambda(SEG) mice produce only lambda2 and lambdax light chains. Regardless of their Igh haplotype, B6.kappa(-)lambda(SEG) mice show a restricted B cell distribution by light chain subtype with lambdax dominance in all peripheral compartments except peritoneal cavity where lambda2 is dominant. This distribution suggests that selection mechanisms act differently in different B cell compartments on lambda2 and lambdax bearing B cells. Sequence analysis before or following immunization did not reveal unusual mechanisms of diversification. B6.kappa(-)lambda(SEG) mice still respond to various challenging antigens using new Ab patterns. In particular, regardless of Igh(a) or Igh(b) haplotypes, the anti-2,4-dinitrophenyl response is characterized by a restricted diversity for both heavy and light chains and a delayed IgG response when compared to B6 and B6.kappa(-) mice. We suggest that the delayed IgG response is due to the expansion of marginal zone B cells whereas follicular B cells are strongly reduced.