This study was performed to determine the effects of a natural yeast-based feed ingredient (natural carbohydrate fraction (NCF) isolated from a specific strain of yeast) on broiler chickens, and to examine its mode of action. The trial was set up as a complete randomized design with three treatments and eight replicates (38 Ross 308 chickens per pen). Two levels of NCF, 0.2 g/kg and 0.4 g/kg, were compared to a negative control. The NCF addition increased body weight during the initial period, but this benefit was lost towards the end of the trial. Feed conversion ratio was improved significantly with 0.4 g NCF/kg (1.79 compared with 1.83 in control group). Mortality was numerically lower in the groups receiving NCF. Significant effects on caecal bacterial population were not found. Intestine length and weight were not affected by treatments, while some changes in intestine histology were found. The area described as the ‘cup’ of mucus-producing cells, representing the quantity of stored mucins, was significantly larger in chickens receiving NCF. Relative weights of the spleen and bursa of Fabricius did not change significantly compared with the control. The NCF can improve performance and affects mucus-producing cells. Full elucidation of the mechanisms of action requires further research.