Effects of tannic acid (TA) on the immune response of growing chickens were studied in two experiments. In the first experiment, 25 or 30 g TA/kg diet was added to a control diet; added TA reduced (P less than .05) weight gain, protein efficiency rate and weight of bursa of Fabricius, thymus and spleen. Total IgM and IgG immunoglobulin levels (primary and secondary immune responses) against killed Brucella abortus were reduced in TA-fed chickens. Numbers of total white blood cells and absolute lymphocytes also were reduced. Tannic acid feeding also resulted in the involution of bursal follicles. In second experiment, TA-treated birds were challenged with a live culture of Brucella abortus. The total IgM and IgG levels again were depressed along with lymphoid organ weights of treated birds. Persistence of B. abortus in spleens of TA-treated birds was higher. These data suggest that the administration of tannic acid impairs the immune function of growing chickens in a dose-dependent manner.