In this work the immune structure of an organized group with respect to acute respiratory infections and the etiological structure of cases of acute pneumonia was studied. The sera taken from 602 persons were tested for the presence of antibodies to 14 infective agents of the viral, bacterial and mycoplasmal nature. The data indicate that all these agents circulate in the group under study with the prevalence of influenza A (H3N2), influenza B and group III parainfluenza viruses. Among etiological agents of acute pneumonia, the combination of Staphylococcus with Mycoplasma pneumoniae and adenovirus was noted. The Legionella etiology of some group cases of acute pneumonia among young persons was established on the basis of the increased titers of L. pneumophila antibodies in the indirect hemagglutination, indirect immunofluorescence and enzyme immunoassay. The source of this infection was highly dispersed water aerosol, containing the infective agent, in shower baths. This was bacteriologically confirmed by the isolation of three Legionella strains. The study showed that no transfer of Legionella infection in the group through contacts was possible.