Listeria monocytogenes (L. monocytogenes) secretes proteins associated with its virulence into the cytosol of infected cells. These secreted proteins are degraded by host cell proteasomes and processed into peptides that are bound by MHC class I molecules in the endoplasmic reticulum. We have found that the MHC class I antigen-processing pathway is very efficient at generating the epitopes that are presented to cytolytic T lymphocytes (CTL). Depending on which antigen is investigated, from 3 to 30% of degraded antigens are processed into nonamer peptides that are bound by MHC class I molecules. Surprisingly, neither the efficiency of epitope generation nor the absolute number of epitopes per infected cell determines the magnitude of the in vivo CTL response. One of the least prevalent epitopes, derived from an antigen that is virtually undetectable in infected cells, primes the immunodominant CTL response in L. monocytogenes-infected mice. Our studies suggest that immunodominant and subdominant T-cell responses cannot be predicted by the prevalence of antigens or epitopes alone, and that additional factors, yet to be determined, are involved.