PE-9 CTLs cultured in the presence of lysates of LSA or EL-4 tumor cells exhibit decreased cell viability. Fas⁺ PE-9 cells were cultured in vitro for 24 hours in the presence of medium alone (control) or lysates of LSA, EL-4, or lymph node cells from gld or lpr mice. Cell viability was measured by trypan blue dye exclusion. Vertical bars represent mean viable cell counts in triplicate cultures ±SD.