CD34⁺CD40⁺ but not CD34⁺CD40⁺ blood cells induce T-cell alloresponses in primary MLC. 2 × 10⁴ purified CD34⁺CD40⁺ and CD34⁺CD40⁻ blood cells, or 5 × 10⁴ autologous MNC were irradiated and tested in primary MLC with 5 × 10⁴ allogeneic mononuclear responders. SI were calculated for each experiment. Results are represented as the mean SI ± SEM. CD34⁺CD40⁺ blood cells induced a significantly higher alloresponse than CD34⁺CD40⁻ blood cells (P = .01) (n = 3 experiments).