A substantial proportion of murine bone marrow B220+ and IgM+ cells were induced to undergo apoptosis when exposed to glucocorticoids or ionizing radiation in vitro. Two-colour flow cytometric analysis of the cell cycle indicated that a distinct subpopulation of cells formed to the left of G0/G1 in the hypodiploid or Ao region previously shown to contain apoptotic cells with fragmented DNA. Indeed, 45-65% of all B220+ or IgM+ cells of the marrow were found in this apoptotic region 12 hr after treatment with dexamethasone (Dex) or exposure to 500 rads of irradiation. Zinc sulphate, a frequently cited inhibitor of apoptosis, prevented accumulation of cells exposed to glucocorticoids or ionizing radiation in the Ao region as did the glucocorticoid receptor antagonist RU 38486. Although Dex was more potent, corticosterone and cortisol also induced significant degrees of apoptosis in B220+ and IgM+ marrow cells at physiological concentrations. These results demonstrate that freshly isolated B-lineage cells of the murine bone marrow readily undergo apoptosis upon exposure to glucocorticoids and ionizing radiation and suggest that apoptosis may play a role in the regulation of lymphopoiesis. The data also show the value of flow cytometry to the study of apoptosis in subsets of cells within a heterogenous population such as the bone marrow which heretofore was exceedingly difficult to evaluate.