All mice were infected with mouse polyoma virus on day 0. On day 28, mice were challenged with either wild type vaccinia virus (VV-WT) or a genetically modified vaccinia virus expressing the polyoma amino acid sequence LT359-368 (VV-LT). Four days after the vaccinia infection, the mice were sacrificed and the titer of vaccinia virus in the ovaries of each mouse was measured by a plaque assay. ACE 10/10 mice are not intrinsically resistant to WT vaccinia virus. However, once immunized with polyoma virus, the ACE 10/10 mice were substantially more resistant to VV-LT vaccinia virus that expressed the major MHC class I epitope of polyoma. The figure shows data from individual mice, as well as the group means and S.E.M.