A study on turkeys was conducted to evaluate the administration of different levels of mannan-oligosaccharide (MOS) (0.1, 0.25 and 0.5%) to a diet without or with an antibiotic (Flavomycin, 8 mg/kg feed). The growth performance as well as caecal development and metabolism indicators of turkeys after 8 weeks of experimental feeding were estimated. No interactions were noted between the contents of antibiotic and MOS in the diet in any of the parameters examined. During 8 weeks of experimental feeding, the feed intake as well as feed conversion ratio were similar in all experimental groups. The turkeys fed a control diet (without MOS) supplemented with antibiotic were the heaviest, but there were no statistical differences between groups. Depending on dietary dose, MOS had a different influence on caecal digesta parameters. The medium level of dietary MOS (0.25%) resulted in the highest caecal pH, dry matter and protein concentrations as well as the bacterial glycolytic activity (including beta-glucuronidase). Compared to other dietary treatments, the highest amount of MOS (0.5%) reduced ammonia concentration and enhanced volatile fatty acids concentration, especially of acetate and butyrate, in the caecal digesta. The medium level of dietary MOS caused a significant enhancement of propionate, iso-butyrate and iso-valerate concentrations in the digesta. The antibiotic addition to a diet resulted in a lack of birds' response.