CD34⁺ cell–mediated suppression of CTL responses is not prevented by addition of anti-CD28 mAb. A 5-day MLR was established in which responder cells were stimulated against allogeneic cells from the CD34+ cell donor with (•, ▴, ▪) or without (○, ▵, □) the addition of CD34⁺ cells. Anti-CD28 mAb was added (▵, ▴, □, ▪) or not added (○, •) at the beginning of the culture at a concentration of 1 μg/mL (A) or 5 μg/mL (B). The cells were then recultured for 7 days under limiting dilution, and the CTL activity was determined by ⁵¹Cr-release assay. Three experiments were carried out; 1 experiment is presented. The CTL-p frequency (f_WM) was calculated as described in “Materials and methods.” The inhibition of CTL-p–f_WM mediated by CD34+ cells in the presence of anti-CD28 mAb is significant (P < .005 and P < .001 for the concentration of 1 μg/mL and 5 μg/mL, respectively). V(f_WM) values were below 5 × 10^-8. Sloping line indicates CTL-p frequency.