CD8+ T cells are normally less numerous than CD4+ T cells in uninfected individuals. Virus infections can result in an especially intense activation of CD8+ T cells_1–5 and there are large increases in the number of circulating CD8+ T cells in humans infected with cytomegalovirus, Epstein-Barr Virus, or Varicella-Zoster Virus._1,3,4 In mice infected with LCMV there is an approximately 5-fold increase in the number of CD8+ T cells.₆ As the total number of CD8+ cells expands, limiting dilution analysis (LDA) reveals that the number of virus specific CTL also increases._5–8 However, LDA indicates that the number of virus-specific CD8+ T cells is small, 1/4000-1/50 depending upon the virus._5–8 The specificities of the rest of the expanding CD8+ T cells has not been determined, though it has been shown that during LCMV infection there is an increase in non-LCMV reactive CTL that are alloreactive and antigen crossreactive CTL._9,10 Furthermore, there are not only more CD8+ T cells during the immune response to LCMV, but most of them appear to have been activated as they are enlarged, have elevated surface expression of CD11a, CD11b, CD44, CD49d, IL-2R and reduced expression of CD62L._11–13