CD8 +/Leu-7 + T cells which circulate in increased proportions in the blood of long-term surviving BMT patients are for the most part high-density resting lymphocytes lacking IL2R-α (p55) expression. We show that they can be induced by IL2 to manifest cytolytic function after 24–48 hr stimulation by using rather high concentrations of IL2 (at least 50 U/ml). This function was much more readily induced in high-density CD8 +/Leu-7 + T cells than in high-density CD8 +/Leu-7 − T cells and occurred in the presence of minimal cell proliferation. Other cytokines involved in primary CTL differentiation (IFN-γ, IL4 and IL6) were without effect suggesting that CD8 +/Leu-7 + T cells are, in the BMT model, in vivo preactivated CTL ready to differentiate into cytolytic effectors under the sole IL2 stimulus. TU27 Mab directed at IL2R-β (p75) subunit almost completely prevented IL2-induced cytolytic function of CD8 + T cells while 33B3.1 Mab directed at IL2R-α (p55) subunit was ineffective, suggesting that the signal for this function has its origin in IL2R-β chains constitutively expressed by these cells.