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: C (unsupplemented), A (supplemented with flavomycin), and P (supplemented with a probiotic composed of lactic acid bacteria, yeasts, and yucca extract). 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. GI tract digesta pH was lower in groups P and A than in the control group, significantly (P<0.05) lower in the crop and ileal content, only. In group P the Lactobacilli and E. coli counts in excreta were higher (P<0.05) in 1- to 3-week old chickens than in group C, while the Clostridium count, lower (P<0.05) than in group A. In the following weeks the tendency was similar, but insignificant. It may be concluded that the studied probiotic can be considered to be a substitute for antibiotic growth promoters in broiler diets.