Glutamine represents the principal metabolic substrate for all rapidly proliferating cells. Since part of the glutamine efficacy could be related to immunoregulating properties, we assessed the effects of orally administered glutamine on serum interleukin-2 (IL-2) levels and intestinal T-cell populations in 48 athymic (nude) mice. Twenty-four mice received a standard diet enriched by glutamine (added to drinking water at a 4% concentration), while the other 24 served as the control group and received the same diet without glutamine. In glutamine-fed animals, we observed a marked increase in IL-2 concentrations after 10 days of treatment in comparison with control group and a modest but significant increase in intestinal T-cell counts. These results suggest that oral glutamine is able to exert local and systemic immunostimulating activity that could be of relevance in the prevention of gut integrity and immune defense loss associated, for example, with trauma, surgery, and starvation.