L3055 cells were incubated with CD40L alone, IL-4 alone, or CD40L with IL-4 for either (A) 24 or (B) 72 hours. Aliquots were removed at the times indicated and cell growth was evaluated by incorporation of [³H]-Thy. Incorporation of [³H]-Thy for each treatment 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 for the three experiments was 7,698 to 11,218 cpm. Treatment with either CD40L alone or CD40L with IL-4 significantly (P < .0001) decreases proliferation of L3055 cells where indicated (*).