Donor T cells lacking CD28 cannot generate cytotoxic effectors against the recipient. Fifteen days after transplantation, splenocytes from normal BDF1 mice and from BDF1 recipients of CD28^+/+ or CD28^−/− cells were assayed directly without in vitro restimulation. Three mice per group were analyzed separately, and the data represent the mean +/− 1 SD of three individual mice. The activity of anti-H2^d or anti-H2^b cytolytic effectors was measured in a 4-hour cytotoxicity assay against specific P815 (H2^d) or control EL-4 (H2^b) targets.