Peripheral B cells are divided into two subpopulations, B-1 and B-2, the relationship of which remains obscure. We recently showed that the Bcmd mutation in A/WySnJ mice reduces average B cell life span, yielding 90% fewer peripheral B cells. Despite this defect, A/WySnJ mice have an elevated proportion of peritoneal CD5+ B cells, suggesting that Bcmd may be the first B-cell-intrinsic gene to differentially affect the B-1 and B-2 subpopulations. To test this hypothesis in detail, we have used in vivo BrdU labeling and four-color cytofluorometry to examine the numbers and turnover rates of sIgM+ CD23− CD43+(B-1) and sIgM+ CD23+ CD43−(B-2) splenocytes in A/WySnJ and A/J mice. The results show the expected 90% reduction of splenic B-2 cells among A/WySnJ mice, but a normal splenic B-1 cell pool. Increased B-1 cell renewal cannot explain this undiminished pool, because BrdU labeling kinetics reveals an identical splenic B-1 subset turnover rate of∼ 4%/day in both A/WySnJ and A/J strains. Thus, B-1 cells are Bcmd-independent but B-2 cells are Bcmd-dependent, suggesting Bcmd functions in a positive signaling pathway that imparts longevity to quiescent B cells, but that is not required for cycling B cells. Moreover these results show that the requisites for maturation and longevity differ between the B-1 and B-2 subsets.