With respect to the fact that acute respiratory infections (hereafter ARI) are a world-wide serious social and economic problem, more and more attention is being paid to the development and application of oral bacterial immunomodulators. The preventive effect of GS Imunostim has been evaluated during the respiratory season 2005. The study took place at 10 outpatient sites of general practitioners and lung specialists in Prague, Pilsen and Brno in the respiratory season from January to April 2005. The comparison with the ARI morbidity reports in the studied period revealed statistically significant morbidity reduction in the study participants compared to the ARI morbidity in the Czech population in the studied period from January to April 2005. The morbidity rate in the studied group was 7%, whereas ARI morbidity 15.1% was reported in the general population. The morbidity in the study group was significantly lower that in the general population.