In-feed antibiotics have been used routinely in animal nutrition for a long time. Nevertheless, their use has been viewed critically and their application has been restricted. It led to a decrease of performance and increase of mortality and morbidity, especially in weaned animals. Fatty acids and their derivatives are among the candidate replacements for antibiotics. Following our previous results, where we observed antibacterial activity of some fatty acids in vitro, it is necessary to confirm these observations in animals. The aim of the proposed project is to evaluate the effect of fatty acids on the health status and performance of broiler rabbits and chickens. Also the mode of action of fatty acids is still not clear. Fatty acids possibly facilitate the transport of protons across the bacterial membrane and thus collapse the membrane potencial. This can lead to the depletion of cellular energy followed by denaturation of acid-sensitive cell structures.