Four late passage BL cell lines were incubated with CD40L alone, IL-4 alone, or CD40L with IL-4 for 72 hours. Aliquots were removed at the times indicated and cell growth was evaluated by incorporation of [³H]-Thy, which is expressed as a percentage of untreated control cultures. Data are presented as the mean with standard deviation from three separate experiments. The range of radioactivity incorporated in control (untreated) cultures in these experiments for each cell line was: Elijah, 12,433 to 23,083 cpm; BL2, 14,053 to 22,767 cpm; BL29, 19,353 to 24,662 cpm; and Louckes, 13,471 to 23,622 cpm. Treatment with CD40L significantly (P < .0001) reduces proliferation of all late passage BL cell lines where indicated (*).