In an experiment performed on 600 ROSS broilers the effect of dietary lactic acid bacteria (LAB), mannan oligosaccharide (MOS) without (LAB, MOS) or with fumaric acid (LAB, MOS, FUA) in comparison with antibiotic-supplemented birds (ANT) on performance, mortality, carcass yield, and microflora spectrum of the digestive tract was studied. The body weight of birds at 42 days was 2.2, 2.5, 2.4 and 2.4 kg, in the CON, ANT, LAB + MOS and LAB + MOS + FUA groups, respectively (P≤0.01). No significant differences between the body weights in the antibioticsupplemented (ANT) and experimental groups were found. Bird mortality in the CON, ANT, LAB+MOS and LAB+MOS+FUA groups was 2.7, 0.0, 0.7 and 0.7%, respectively. Compared with the control group, use of the antibiotic or other additives increased feed intake by 0.49 kg/bird, on average. Dressing percentage averaged 73.5% and was significantly higher in the experimental groups (P≤0.05). The weight of cold carcasses averaged 1.8 kg and was significantly lower in group CON (P≤0.01). The pH of the digestive tract did not differ among groups. LAB and MOS with or without FA increased significantly (P≤0.05) the counts of Streptococcus (P≤0.01) and Lactobacillus in the ileal digesta. The Streptococcus and Escherichia coli counts in the caecal digesta decreased significantly (P≤0.01). No Salmonella, Shigella and Campylobacter were found in the small intestine or caecum of any of the groups.