One thousand five hundred and forty Hybro HI-YR broilers were used in the study. One-day-old chickens were allocated to four groups; in the pen trial each group comprised 5 replications with 56 birds, in the digestibility trial there were 7 replications with 15 birds. The dietary treatments were as follows: unsupplemented control wheat-barley diet or the same diet supplemented with 10 ppm avilamycin, 150 ppm or 300 ppm of plant extract (XT) containing capsaicin (1.98 g/100 g), carvacrol (4.95 g/100 g) and cinnamic aldehyde (2.97 g/100 g). Performance was determined on days 21 and 48. After slaughter the rectum content was collected for E. coli and Clostridium perfringens assay; slaughter yield and sensorial evaluation of breast and thigh muscles were performed on 16 birds per group. In the digestibility trial, on days 21 and 32 of life, after slaughter the apparent ileal digestibility of nutrients and amino acids was estimated on 28 birds per group. In comparison with the control group, up to day 21 of life the administration of feed antibiotic and 150 ppm or 300 ppm of plant extract enhanced body weight by 4.7, 5.4 and 8.1% (P<0.01) and improved the feed conversion ratio by 5.8, 3.1 and 7.1%, respectively (P<0.05). The mortality and culling rate was low (1.8-2.5%). At 48 days of age, performance in groups as well as carcass characteristics were less diversified, only a slightly higher breast muscle share was obtained in the experimental groups. Sensorial characteristics of the breast and thigh meat were best in the group receiving 300 ppm XT. In all supplemented groups the number of E. coli and Clostridium perfringens in rectal contents was significantly lower than in the control group; supplementation of the diet with 300 ppm XT and feed antibiotic had a similar effect on the number of evaluated microorganisms. Apparent ileal digestibility of nitrogen, nutrients, and amino acids, particularly of threonine, serine, asparagine, phenylalanine, histidine and lysine, in 21-day-old chickens was positively affected by XT supplementation. The coefficients of apparent ileal digestibility of nutrients in older birds (32-day-old) in groups fed diets with XT or feed antibiotic were similar.