1. In experiments carried out with 950 one-day-old male chickens, the effect of tannin supplementation (0, 250, 500 and 1000 mg/kg) on performance, microbial status of chickens small intestine and colon of 28- and 41-d-old chickens, as well as histological changes of jejunum walls at 41 d and carcase quality were determined. 2. Application of 250 or 500 mg of sweet chestnut tannin per kg of feed had an insignificant effect on body weight and feed conversion of 41-d-old chickens (30 and 26%) in comparison to control birds. The highest tannin supplement (1000 mg/kg) reduced final body weight. 3. No effects of tannin supplementation on feed conversion and carcase quality were found. 4. Addition of tannin increased dry matter content of the litter by 88 (Group II) and 77% (Group III) when compared to control. 5. Higher doses of tannins significantly reduced the number of E. coli and coliform bacteria in small intestine of 28-d-old chickens; in other microorganisms great variability of microbial populations in small intestine and colon were observed. 6. The histologies of jejunal walls in chickens of control, II (250 mg/kg) and III (500 mg/kg) groups were similar. The structure was characteristic of correctly developed and functioning tissues and the villi were formed correctly. Tannin applied at the highest dose (1000 mg) slowed down the proliferation rate in the mother-cell zone. Single cells and enterocyte complexes showed the features characteristic of degradation processes. These unfavourable symptoms indicated some disturbances in intestinal wall morphology and function.