The experiment was carried out on 99 female broilers, allocated to 3 groups of 33 birds, kept in individual cages. A wheat- and soyabean meal-based diet was formulated and divided into 3 batches: P (supplemented with a probiotic composed of lactic acid bacteria, yeasts and yucca extract), A (supplemented with flavomycin) and C (unsupplemented). Diets were fed from day 1 to 41 of life. Final BW was on average 2.4 kg in all groups, FCR was 1.63 kg feed/kg BWG, neither BWG nor FCR nor mass of the liver, pancreas and gastrointestinal tract were significantly influenced by supplementing the diet with either additive. Special attention should be drawn to the fact that supplementing feeds with a probiotic preparation already after one week of breeding considerably decreased the number of Clostridium bacteria in broilers’ faeces. Nevertheless, it should be emphasized that in the hereby research excreta of chickens fed with the feed mixed with a probiotic contained the lowest changeability of the number bacteria of Enterobacteriaceae family and bacteria belonging to coli group in individual weeks of breeding. It may be concluded that the studied probiotic can be considered to be a substitute for antibiotic growth promoters in broiler diets.