In the present study we have addressed the question of whether the composition of the peripheral B cell pool is similar in mice raised under conventional, specific pathogen-free or germ-free breeding conditions. For this purpose, the representation of various B lineage subsets was quantitated by flow cytometry and the lifespan of B cells analyzed by labeling dividing cells with the thymidine analogue 5-bromo-2'-deoxyuridine in vivo. The results indicate that except for an increased fraction of CD45R (B220)dull B lineage cells in the bone marrow and a marginally faster turnover of splenic B cells in germ-free compared to conventional mice, there is no obvious difference in the representation and turnover of B cells in the different kinds of mice. The data indicate that a stable pool of peripheral B cells is generated even in the absence of microbial stimulation.