One of the most frequently reported immunomodulatory actions of n−3 PUFA is their ability to diminish in vivo lymphocyte proliferation. The purpose of this study was to determine if n−3 PUFA intake affects the kinetics or magnitude of the antigen-driven expansion of CD8+T-lymphocytes in vivo. In this study we utilized a well-characterized model of T-cell immunity (i.e. infection with the intracellular bacterium, Listeria monocytogenes). Weanling BALB/c mice were fed one of two experimental diets that differed solely in fat source. Our control diet contained lard (180g/kg) and was devoid of long-chain n−3 PUFA. The experimental diet contained 150g/kg menhaden fish oil and 30g/kg corn oil, thus providing approximately 8% of energy from long-chain n−3 PUFA. After 4 weeks, mice were infected intravenously with 106 colony-forming units of actA-deficient L. monocytogenes. Clonal expansion of antigen-specific CD8+T-cells in the spleen was measured at 5, 7, 9 and 14d post-challenge using a class I MHC tetramer loaded with the immunodominant peptide from this pathogen (i.e. Kd:LLO91–99). We report that feeding mice a diet rich in n−3 fatty acids did not significantly impact either the kinetics or magnitude of in vivo, antigen-driven expansion of CD8+T-cells. Furthermore, contraction of this T-cell population was not affected by n−3 PUFA treatment. To our knowledge this is the first time MHC tetramers have been used to investigate the influence of n−3 PUFA on in vivo CD8+T-cell proliferation.