The TNFR family member BAFF-R facilitates peripheral B cell development, although it is unclear whether it promotes survival of B cells, or also initiates a differentiation program. We show that disruption of the BAFF-R encoding gene Tnfrsf13c in strain A/WySnJ mice causes a progressive decline in peripheral B cell numbers, beginning at the transitional 1 developmental stage and continuing through the mature peripheral B cell stage. Bcl-x L overexpression in A/WySnJ B cells decreased the turnover of transitional B cells, as determined by 5-bromo-2′-deoxyuridine labeling, and restored follicular B cell development. We conclude that the mutant A/WySnJ allele of Tnfrsf13c can be complemented through the survival signal provided by Bcl-x L.