Neutralization of IFN-γ in culture media accelerates spontaneous apoptosis of EBV-infected NK leukemia cells in vitro. Data are shown as mean OD values on MTT assay 24 hours after the culture that correlate with viability of the cells. In all 3 cases studied, the addition of neutralizing anti–IFN-γ antibodies to the culture media resulted in an accelerated loss of viability of NK leukemia cells in vitro. *A significant difference between the groups (P < .05) determined on Student’s t-tests.