The aim of the study was to determine the antibacterial effect of a commercial product containing medium-chain fatty acids on Campylobacter jejuni. The product was tested in vitro, followed by evaluating the effect by experimental infection. Broiler chickens were artificially infected with C. jejuni and fed with mixture containingthe additive Fortibac® (Delacon, CR). The final concentration of the additive was 0, 0.25 and 0.5% of the feed mixture. The feed was used during the entire fattening period (35 days). During the experiment,feces were collected every two days and microbiologically analyzed. Reduction of the number of campylobacters by the product was not confirmed. 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 positive impact on mortality of infected chickens.