Previous studies have demonstrated that dietary lipid manipulation may modify immune response by affecting lymphocyte proliferation, phagocytosis, cytokine production, etc. In this paper, we investigated the effect of olive oil (00) on, the phagocytic activity and cytokine production by murine peritoneal cells. These results were compared with those obtained from mice fed diets containing sunflower oil (SO) or hydrogenated coconut oil (HCO). Balb/c mice were divided into three groups and fed diets containing 15% by weight of either 00, SO or HCO for 5, 15, 30, 60 or 90 d. Phagocytic activity and interleukin-1 (IL-1) production were increased in 00-fed mice as compared to the other groups. On the contrary, io significant differences were observed in the levels of tumor necrosis factor (TNF) production, although the levels of this cytokine were slightly increased in mice fed the 00 diet. These observations suggest that 00 is able to modify the immune response and therefore, it may be used as an immunomodulatory agent.