In this study, the effect of bleomycin on myeloid leukemic U937 cells transfected with murine bcl-2 or vector alone (vector containing neomycin-resistant gene only; MNC) was investigated. Sublethal concentrations of bleomycin (1 microgram/ml) induced a decrease in cell growth in both vector-only and bcl-2-transfected U937 cells. In MNC-transfected U937 cells, loss of viable cells and colony-forming cells was observed following 4 days of bleomycin treatment. This was accompanied by accumulation of cells in the G0-G1 phase of the cell cycle and morphological changes as well as induced expression of markers associated with myeloid differentiation (i.e., increased granularity and CD11b expression). In contrast, bcl-2-transfected U937 cells maintained viable cell numbers and colony forming cells for up to 2 weeks in the presence of bleomycin. These cells did not show cell cycle accumulation in G0-G1 and in addition, displayed delayed expression of differentiation markers when compared with bleomycin-treated, vector-only transfected U937 cells. One day following a 5-day exposure to 1 microgram/ml bleomycin, a loss of differentiated cells by apoptosis, as demonstrated by dUTP and analyzed by flow cytometry, was observed in the MNC-transfected U937 cell population. In contrast, differentiated bcl-2-transfected U937 cells remained viable for 2 weeks following bleomycin treatment. The results of this study suggest that up-regulated Bcl-2 not only blocks apoptosis in proliferating myeloid cells but also delays or prevents apoptosis in differentiated myeloid cells.