Some studies have reported increased apoptosis in CD8 + T cells from aged mice. We previously demonstrated diminished virus-specific CD8 + cytotoxic T lymphocyte (CTL) activity in aged mice in comparison to young mice. The present study investigated the role of apoptosis in age-related influenza virus-specific CD8 + CTL deficiency. Splenocytes from influenza-primed aged and young mice were stimulated in vitro with virus. The CD8 + T cell/total lymphocyte ratios correlated with CTL activity and were significantly decreased and increased in aged and young mice, respectively. Fas, FasL, TNF-a and TNFR-p55 expression, measured by flow cytometry, ELISA and/or RT ± PCR, were significantly elevated in aged mice. Apoptotic CD8 + T cells (Annexin V binding) were also elevated in aged mice. IL-12 treatment increased CD8 + CTL activity and IFN-g production but did not affect apoptosis. Thus, apoptosis may contribute to reduced influenza virusspecific CD8 + T cell frequency, CTL deficiency and increased influenza disease in aging.