A 2 x 3 factorial arrangement of treatments was used to determine the effects of olaquindox and cyadox on the intestinal mucosal immune response and on fecal shedding of Escherichia coli in Landrace x Large White barrows that had been orally given 10(10) cfu of E. coli (O139:K88). Factors included 1) E. coli inoculation or no inoculation, and 2) no antimicrobial, 100 mg of olaquindox/kg, and 100 mg of cyadox/kg in the basal diet, respectively. The effects of cyadox and olaquindox were assessed in terms of fecal shedding of E. coli, the number of intraepithelial lymphocytes (IEL), immunoglobulin A-positive cells (APC) in the intestinal lamina propria, and ADG. There was no difference in the fecal shedding of total E. coli or the inoculated E. coli between olaquindox-supplemented pigs and cyadox-supplemented pigs during the experiment. However, fecal shedding of the inoculated E. coli in olaquindox- or cyadox-supplemented pigs was less (P < 0.05) than that in nonsupplemented pigs. Escherichia coli inoculation increased IEL and APC in the jejunum and ileum, but olaquindox or cyadox decreased IEL and APC (P < 0.05). Jejunal APC in cyadox-supplemented pigs was less (P < 0.05) than that in olaquindox-supplemented pigs. Escherichia coli inoculation reduced (P < 0.05) ADG, whereas the supplementations improved ADG (P < 0.01) during the experiment. Average daily gain in cyadox-supplemented pigs was greater (P < 0.05) than that in olaquindox-supplemented pigs. The data indicated that olaquindox and cyadox reduced the number of intestinal E. coli and suppressed E. coli-induced immune activation, which might be responsible for the enhanced growth that was observed.