Dietary (n-3) fatty acids from fish oil have been used to modulate immune function in many mammalian species. Together, dietary antioxidants and behavioral enrichment have been shown to enhance neutrophil phagocytosis in geriatric Beagle dogs. The purpose of this study was to further investigate the effects of vitamins E and C, in combination with dietary fish oil, on neutrophil mediated bacterial killing, and on transcript levels of selected neutrophil mRNA. Fifty adult Beagle dogs were randomized into five dietary treatment groups for 60 days. All foods were complete and balanced and met the nutrient profiles of AAFCO for adult dogs. For 60 days before study initiation, dogs consumed a pretrial food that contained 74 IU/kg vitamin E and 0 mg/kg vitamin C. The five experimental foods were confirmed by analytical methods to contain ≥640 IU/kg vitamin E and 130 mg/kg vitamin C (as fed). Experimental foods ranged from low levels of EPA and DHA (pretrial food and lowest experimental food had 0.01% EPA and no detectable DHA) to the highest experimental food with 0.25% EPA and 0.17% DHA. Ex vivo bactericidal activity of activated, peripheral-blood neutrophils against Lactococcus lactis was determined after 1 h incubation. Bactericidal activity was calculated as a percentage of control values (bacteria incubated in media without neutrophils). Transcript levels of genes involved in neutrophil-mediated immune functions were determined by real-time qPCR. Dogs in all treatment groups had increased serum vitamin E concentration (P<0.01). After consuming experimental food for 60 days, neutrophils from dogs in all 5 treatment groups also had increased bactericidal activity (P<0.01). Dietary fish oil however, had no effect on bactericidal activity. Stepwise multiple regression analysis demonstrated that the change in neutrophil mediated bacterial killing was significantly correlated to changes in gene expression of interleukin-8 receptor (IL-8R), interleukin converting enzyme (ICE), and myeloperoxidase (MPO; r(2)=0.33; P=0.003). When stepwise multiple regression analysis was performed considering each mRNA as a dependent variable and change in selected individual and summed fatty acid concentrations as independent variables, change in the ratio of saturated fatty acids (SFA) to polyunsaturated fatty acids (PUFA) was significant (P≤0.05) in the mRNA regression analyses for IL-8R, ICE, MPO, and cyclooxygenase-2. In summary, circulating neutrophils from dogs fed diets enriched in vitamins E and C had significantly increased bactericidal activity as well as altered gene expression. Change in SFA to PUFA ratio also altered neutrophil gene expression.