Increased tumor-specific CTL activity in mice bearing L1210 Fas⁺ but not L1210 Fas⁻ cells. Purified T cells from DBA/2 mice bearing L1210 Fas⁺ or L1210 Fas⁻ tumor cells were obtained and cultured with irradiated L1210 Fas⁺ and Fas⁻ tumor cells, respectively, for 5 days. Next, the harvested cells were tested for cytotoxicity against L1210 Fas⁺ or Fas⁻tumor targets (A) or against P815 (B) as described in Fig 4. In the symbols, the first column represents source of T cells, the second shows stimulator cells, and the third depicts target cells tested. For example, Fas⁺ / Fas⁺ / Fas⁺represents T cells from mice bearing Fas⁺ tumor cells stimulated with Fas⁺ tumor cells in vitro and tested for cytotoxicity against Fas⁺ targets.