The aim of the study was to determine the effects of supplementation of sows’ and weaners’ diet with Stresomix, preparation containing extracts from Magnifera indica, Withania somnifera, Phyllanthus emblica, and Ocimum sanctum, on pig performance and immunity under field condition. The hypothesis was that anti-inflammatory, antistress, and immunomodulatory properties of the herbs would enhance production parameters and immune response, according to the manufacturer's claim. The study was performed on 16 sows and 160 piglets. The following parameters were recorded: concentration and proportion of white blood cells and their populations, concentration of serum immunoglobulins, specific humoral postvaccinal response after vaccination against swine influenza and swine erysipelas, and main production parameters. No significant differences among treatment groups were found with regard to concentrations of leukocyte subpopulations and immunoglobulins, as well as all investigated production parameters (P>0.05). In conclusion, the results of the study did not confirm that the investigated polyherbal product, administered at dose recommended by manufacturer, is able to significantly improve the performance and postvaccinal humoral response in clinically healthy pigs under field condition.