CD34⁺ cell–mediated suppression of CTL responses is prevented by the caspase inhibitor BD-FMK. 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. In some cultures, the responder cells were precultured for 2 hours with the caspase inhibitor BD-FMK (500 μM) and were then added to the MLR. The BD-FMK inhibitor was also added to the MLR at a final concentration of 250 to 375 μM. The CTL activity was determined by the end of 7-day limiting dilution cultures. Data show the average (± standard deviation) of 3 experiments at cell concentrations of 4 × 10⁴ and 1 × 10⁴ cells per well, calculated as described in “Materials and methods.” *Statistically significant (P < .05 when compared by t test with control cultures without CD34+ cells).