The inhibitory properties of a commercial product Fortibac® containing medium-chain fatty acids on Campylobacter jejuni were determined.The product is a mixture fatty acids. After testing the antibacterial properties towards C. jejuni in in vitro conditions, an experimental infection on broiler chickens was performed to confirm the results. The product was admixed with feed (final concentrations 0, 0.25. and 0.5%) and broiler chickens were artificially infected with C. jejuni VFU 612. The chickens were infected on day 16 of age. while the aforementioned feed mixtures were used during the entire fatten­ing period (days 0-35). After the infection, the dynamics of C. jejuni shedding was evaluated among treated groups and the control. Reduction of the number of Campylobacters by the product with medium-chain fatty acids was not confirmed in vivo. It is assumed that the final amount of potentially active fatty acids in the digestive tract was not sufficient. The product, how­ever, had a clear beneficial impact on mortality of infected chickens.