The aim of the study was to assess the safety of three newly developed synbiotic preparations in feeding chickens. The study was carried out on 84 Specific Pathogen Free chickens, between 1st and 21st day of their life. Animals were divided into four groups fed ad libitum: three with an addition of synbiotics A, B or C and a control group (feed with no additives). Synbiotics contained Lactobacillus, Saccharomyces cerevisiae and inulin (prebiotic). Rearing parameters were determined taking into account undesirable clinical and sectional symptoms, daily mortality and body weight of birds. In addition, chicken blood parameters were determined based on haematological, biochemical and serological tests. Samples of the intestinal content and of the excreta constituted a material for research aimed at determination of the dominant chicken gut microbiota. On the basis of the conducted investigation, it was found that synbiotics met the basic requirements for this type of formulas regarding the safety of use and had a positive effect on the health of chickens. Therefore, further research is being carried out on the application of these formulas in large-scale production. The next step of the research will be the comparison of effects of synbiotics with commercial probiotics.