L3055 cells were incubated either alone or with CD40L, IL-4, or CD40L with IL-4 for 72 hours. Cells were then maintained in complete medium for up to 14 days (including the initial 72 hours of stimulation). During this time at (A) 3 and (B) 14 days of resistance to apoptosis induced by anti-IgM MoAb, BU1 was assessed by counting apoptotic cells from triplicate cultures on Romanoski-stained cytocentrifuge preparations. Data are representative of two separate experiments.