Subsets of CD21^lo transitional, CD21^hi transitional, naive, and memory B cells were purified from the PB of healthy donors and then cultured (5-15 × 10³ cells per 100 μL per well) in the absence or presence of either F(ab′)₂ fragments of anti-Ig (2.5 μg/mL), CD40L (1/200 dilution of membrane preparation), or anti-Ig plus CD40L (Culture conditions #1); or CD40L alone or together with IL-4 (100 U/mL), IL-10 (100 U/mL), IL-13 (10 ng/mL), or IL-21 (50 ng/mL; Culture conditions #2). Proliferation was assessed by pulsing each culture well with 0.5 μCi of [³H]thymidine and determining its incorporation during the final 18 hours of a 5-day culture. The results represent the mean ± SEM of replicate cultures (3 independent experiments for anti-Ig/CD40L and 2 independent experiments for cytokine).