It is important to recognize that the ICS assay calculates the percentage of IFN-γ + cells among CD3 + T cells in the spleen (4-5%), whereas the ELISPOT assay assesses the number of IFN-γ + cells among all splenocyte cell types (0.05%), and cannot definitively assign the production of IFN-γ to a particular cell type. Thus, the two assays use different denominators in calculating the frequency of IFN-γ + production by splenocytes. Importantly, each method clearly identified K 93 FITSRCRL and F 57 GYMTFVHF as the only two peptides from a panel of 27 that elited significant IFN-γ production from the splenocytes of vaccinated mice. In this study we used BALB/c mice as a model system to identify CTL epitopes in the M protein of PRSSV to circumvent limitations derived from shortages of inbred pigs and a paucity of reagents to evaluate porcine immune responses. The identification of PRSSV CTL epitopes in BALB/c mice allow for the future generation of reagents, such as MHC class I: peptide staining reagents, that will enable the in-depth investigations of CD8 + T cell responses during PRSSV infection and immuinization. Whether these two epitopes can bind the SLA class I molecules of pigs remains to be determined. In some cases, specific peptide epitopes are known to be recognized by cytotoxic T cells in different animal species. For instance, the core region of HCV contains an epitope that is recognized by cytotoxic T cells of both mice and humans [19] . Further research on the role of these peptide epitopes in different species is ongoing in our laboratory.