Virus-specific CD8 T cells present in latently-infected mice retain CTL function with age. Cytolytic activity of ORF61-specific CD8 T cells was measured at the indicated times post infection using a 16-17 h (Panel A) or a 4 h (Panel B) in vivo CTL assay as described in the Materials and Methods. Symbols represent specific killing calculated for individual mice. Bars indicate the medians calculated from data compiled from three independent experiments. Significance was assessed using the Mann-Whitney rank test (two-tailed, 95% confidence).