CD34⁺ cell–mediated suppression of CTL responses is prevented by addition of anti–TNF-α but not by the addition of anti–TGF-β1. A 5-day MLR was established in which responder cells were stimulated against allogeneic cells from the CD34+ cell donor in the presence (▨) or absence (▪) of CD34⁺ cells. Anti–TNF-α or TGF-β1 neutralizing mAbs were added at the beginning of the MLR at a concentration of 5 μg/mL. The CTL activity was determined by the end of 7-day limiting dilution cultures. Data show the percent responding cultures at cell concentrations of 4 × 10⁴ and 2 × 10⁴ cells per well, calculated as described in “Materials and methods.”