Cell death in U-1958 cells in the absence or presence of anti-Fas antibodies (CH-11; 100 ng/mL) or control IgM (100 ng/mL). U-1958 cells were preincubated for 96 hours without or with IFN- (1,000 U/mL) or IFN-γ (1,000 U/mL) before the addition of anti-Fas MoAbs or isotype-specific MoAbs. At 24 hours, the cells were harvested and subjected to TUNEL and flow cytometry analysis. Bars indicate the SEM.