Limonene is a monoterpene present in citrus fruit and is used as flavouring agents of foods. It was shown that monoterpenes possess antioxidant activity. Previously, it was demonstrated that limonene exerts anti-proliferative action on a lymphoma cell line without modifying normal lymphocyte viability. H(2)O(2) has a modulator role in cell proliferation. In the present study, the effect of limonene on normal lymphocytes proliferation and its relation with H(2)O(2) level modulation was analysed, evaluating its effect on the activity of cell antioxidant enzymes, such as catalase, peroxidase and superoxide dismutase. Limonene exerted a biphasic effect on cell proliferation; the increase in cell proliferation was related to the decrease in H(2)O(2) level by the increase in catalase and peroxidase activities. Moreover, limonene protected the cells to the oxidative stress induced by exogenous addition of H(2)O(2). In view of these results, it is possible that limonene could protect normal lymphocytes from diseases related to oxidative stress, including cancer, but further research is necessary to establish the role of limonene as a potential antioxidant that can effectively protect lymphocytes from oxidative stress and mitochondrial dysfunction.