Triggering of CD40 enhances CD34⁺ blood cell alloantigen-presenting capacity. Purified CD34⁺blood cells were irradiated (3,000 cGy) and mixed with 5 × 10⁴ allogeneic nylon-wool-purified T cells at 1:2 ratio with or without an anti-CD40 MoAb (B-B20) that is not mitogenic on T cells (data not shown), or an isotype-specific irrelevant control, and cultured for 6 days in MLC. No difference was observed in the response to CD34⁺ blood cells incubated without antibody or with isotype control. Autologous and third-party MNC were used as negative and positive controls. SI were calculated for each experiment. Results are represented as the mean SI ± SEM of 4 separate experiments. B-B20 MoAb significantly increased CD34⁺ blood cell alloantigen-presenting activity (P = .02).