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We have observed that immature B cells (IgM low IgD -) in the bone marrow of adult BALB/c mice exhibit heterogeneity, with a distinct subpopulation (˚4-10%) expressing the CD43/S7 surface protein. These CD43/S7 + immature B cells often express other surface antigens associated with B cell activation (CD5, CD11b, PD-1). Generation of optimal numbers of CD43/S7 + immature B cells requires expression of a functional Btk protein, consistent with activation as a requisite for the CD43/S7 + immature B cell phenotype. Like typical CD43/S7 -immature B cells, the CD43/S7 + immature B cells are predominantly resting cells, which are derived from cycling bone marrow B cell precursors. The CD43/S7 + immature B cell population exhibits enhanced survival in vivo upon administration of the apoptosis-inducing corticosteroid, dexamethasone. Finally, CD43/S7 + immature B cells show a fourfold increase in incidence of VhS107 ? heavy chain expression compared to the CD43/S7 -immature B cells. Therefore, in adult murine bone marrow, the presence of a phenotypically distinct immature B cell population can be demonstrated which has undergone partial activation leading to increased survival and BCR-dependent Vh repertoire selection.