L3055 cells were incubated either alone or with CD40L, IL-4, or CD40L with IL-4 for 72 hours. Stimulants were washed out and cells were maintained in complete medium for up to 20 days (including the initial 72 hours of stimulation). [³H]-Thy incorporation was assessed after a 24 hours of incubation with anti-IgM MoAb BU1 at (A) day 14 and (B) day 20. The percentage of incorporation [³H]-Thy for each treatment was determined from untreated control cultures, which ranged from 4,674 to 8,522 cpm in three experiments. Data are presented as the mean with standard deviation from these three separate experiments. Treatment with either CD40L alone or CD40L in combination with IL-4 significantly (P< .0001) increases L3055 cell proliferation after stimulation with anti-IgM MoAb BU1 where indicated (*).