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Young mice lacking CD28 have normal numbers of peripheral B cells; however, abnormalities exist in the humoral immune response that may result from an intrinsic defect in the B cells. The goal of this study was to assess whether CD28 could be involved in the development of B cells. CD28 mRNA was detected preferentially in the fraction of bone marrow enriched for stromal cells. Flow cytometry and RT-PCR analysis demonstrated that CD28 was also expressed by primary-cultured stromal cells that supported B lymphopoiesis. Confocal microscopy revealed that in the presence of B-lineage cells, CD28 was localized at the contact interface between B cell precursors and stromal cells. In addition, CD80 was detected on 2-6% of freshly isolated pro- and pre-B cells, and IL-7 stimulation led to induction of CD86 on 15-20% of pro- and pre-B cells. We also observed that stromal cell-dependent production of B-lineage cells in vitro was greater on stromal cells that lacked CD28. Finally, the frequencies of B-lineage precursors in the marrow from young (4- to 8-wk-old) CD28(-/-) mice were similar to those in wild-type mice; however, older CD28(-/-) mice (15-19 mo old) exhibited a 30% decrease in pro-B cells and a 50% decrease in pre-B cells vs age-matched controls. Our results suggest that CD28 on bone marrow stromal cells participates in stromal-dependent regulation of B-lineage cells in the bone marrow. The localization of CD28 at the stromal cell:B cell precursor interface suggests that molecules important for T cell:B cell interactions in the periphery may also participate in stromal cell:B cell precursor interactions in the bone marrow.