ABSTRACTObjective. To evaluate the effect of different levels of L-glutamine on weaned and immunologically challenged piglets with Escherichia coli lipopolysaccharides (LPS) on performance parameters, serum cortisol and defense cells. Materials and methods. Four levels of L –glutamine were evaluated (0, 1.0, 1.5, 2.0%) as well as the addition, or no addition, of LPS (0.3μg). 96 piglets were used (48 castrated males and 48 females) of Agroceres x PenArlan lineage, with an initial age of 21 days and 6.06±0.852 kg live weight. An experimental design was used on randomized blocks in a factorial setting 4 x 2 (levels of L- glutamine with or without challenge). Results. Cubic effect was shown for daily weight gain of unchallenged animals, and was better with the addition of 0.41% L- glutamine. Feed conversion improved with increased levels of L -glutamine for challenged animals. In the evaluation of defense cells, there was interaction of leukocytes with the levels of L- glutamine and the immune challenge. Eosinophils and lymphocytes showed a quadratic effect for the levels of L –glutamine, with a maximum value of 1.30% and 0.5%, respectively. Conclusions. L -glutamine supplementation of up to 2% in the diet improves feed conversion and favors the immune serum of weaned piglets challenged with LPS of E. coli.