To examine the relationship between the expression level of Bfl-1 and the susceptibility to staurosporine-induced apoptosis in B-lymphoblastic cells, we tested cell survival in 4 cell lines: 2 that are Bfl-1-nonexpressing (Reh and JM-1) and 2 that are Bfl-1-expressing (IM-9 and Wil2-NS). Reh and JM-1 showed apoptosis levels of 62% (Reh) and 30% (JM-1) as early as 3 hours after treatment with staurosporine, whereas IM-9 and Wil2-NS showed apoptosis levels of only 40% and 26%, respectively, even after 1 day of treatment with staurosporine. Either induced expression of Bfl-1 with 12-o-tetradecanoyl phorbol-13-acetate or exogenous expression of Bfl-1 by transfection in Reh cells promoted cell survival. These results suggest that expression of Bfl-1 contributes to regulating apoptosis in the B-cell lines we examined.