The alloreactive cytotoxic response mediated by LEW CD8 T cells is mainly confined to the CD45RC^high CD8 T-cell subset. Purified CD8 CD45RC subsets (10⁶ cells/mL) from LEW rats were differentiated into effector cells by in vitro stimulation with allogeneic BN or (LEW × BN) F1 splenocytes (5 × 10⁵/mL). Differentiation was carried out for 4 days in the presence of either exogenous IL-2 (A-B) or autologous CD4 T cells (C-D). Thereafter, effector cells were tested for their ability to kill ⁵¹Cr-labeled BN fibroblasts as described in “Materials and methods.” The contribution of MHC class I molecules in this cytotoxic response was tested by adding OX18, an anti-rat MHC class I mAb. (A, C) Results were expressed as means ± SD of the percentage of specific lysis at graded effector-target ratios. (B, D) Results were represented as the percentage of specific lysis at an effector-target ratio of 100:1. Bars represent the mean values of 6 (B) or 3 (D) individual experiments, and filled circles represent the results of each individual experiment. The statistical results of group comparisons (Mann-Whitney U test) are indicated above with horizontal lines.