Abstract: A study was conducted to evaluate the response of broilers to diets containing a mannan oligosaccharide, antibiotics, or a combination of antibiotics and mannan oligosaccharide. All diets were supplemented with copper sulfate to provide 250 mg/kg Cu in diets fed to 42 d and 62.5 mg/kg Cu in diets from 42 to 56 d, in addition to the 10 mg/kg provided in the trace mineral mix. Bio-Mos®, a mannan oligosaccharide derived from the cell wall of the yeast Saccharomyces cerevisiae, was added at 1 g/kg in diets fed to 42 d and at 0.75 g/kg in diets fed 42 to 56 d. The antibiotic program consisted of 55 mg/kg of bacitracin methylene disalicylate to 42 d of age followed by 16.5 mg/kg virginiamycin to 56 d of age. When the Bio-Mos® and antibiotics were fed in combination, half the levels indicated above were fed. Twelve pens of 50 male broilers were fed each of the dietary treatments. Results of the study indicate that body weight of broilers was not significantly influenced by the antibiotic treatment, addition of Bio-Mos®, or the combination of antibiotics and Bio-Mos®. Feed conversion at 42 d was significantly improved by both the antibiotic treatment and by the addition of Bio-Mos®. At 56 d the feed conversion of birds fed the antibiotics or the combination of antibiotics and Bio-Mos® was improved compared to that of birds fed the negative control (P = 0.10). No significant effects on mortality, dressing percentage, or parts yield were observed. Possible interference of copper sulfate with the activity of the antibiotics and Bio-Mos® is discussed.