A study was conducted to determine changes that occur in immune function during the early post-weaning period and the effect of supplementing glutamine (gln, 4% w/w) to the weaning diet of piglets. Dutch-Landrace piglets (n=10/group) were killed prior to weaning (21 d) or randomized to one of two nutritionally complete weaning diets with or without gln. With age there was an increased ability of peripheral blood mononuclear cells (PBMC) and mesenteric lymph nodes (MLN) cells to proliferate (rate of (3)H-thymidine uptake) to T cell mitogens (P<0.05). PBMC from older piglets produced less of a Th-1 type response after stimulation (P<0.05). Adding gln to the weaning diet significantly (P<0.05) modified immune cells in the MLN, in a potentially beneficial manner (with respect to mucosal infections) by preventing an increase in antigen naïve CD4+ cells, increasing the proliferative response to pokeweed mitogen and supporting a Th-1 type cytokine response after T cell (phytohemagglutinin) stimulation.