OBJECTIVE Epidemiological studies have indicated that high intake of w-3 fatty acids influence various diseases such as cardiovascular diseases and autoimmune disorders. These fatty acids are essential in the diet since the body can not form them de novo. Fish oil is rich in w-3 fatty acids but the w-3 content of vegetable oil is low. The research group has shown increased survival of mice fed cod liver oil enriched diet versus mice fed corn oil enriched diet when infected with Klebsiella pneumoniae intramuscularly. In the present study we investigated the effect of dietary fish oil on bacterial growth in vivo. METHODS Mice were fed fish oil enriched diet and a control group was fed corn oil enriched diet for six weeks and then the mice were infected with Klebsiella pneumoniae intramuscularly. The mice were sacrificed at various time intervals and bacteria were counted in blood and in the infected muscle. RESULTS The bacteria count in blood and tissue was not significantly different between the two groups although a trend was noted towards more growth in the control group. CONCLUSIONS We conclude that fish oil does not significantly affect bacterial growth in vivo. Hopefully, future research will reveal the pathophysiological effect of fish oil.