As many as 356 patients afflicted with acute pneumonias (of these, 225 were from 60 to over years of age) underwent examinations according to the program including bacteriological analysis of the mucus and detection of antigens and specific antibodies in the blood serum. The etiology of acute pneumonia was established in 346 patients (97.2%). In the etiological structure of acute pneumonia, there prevailed bacteria, among which pneumococci, streptococci and hemophilic bacilli occurring both in the form of monocultures and as a constituent part of associations were mostly demonstrable. Acute pneumonias caused by Legionella, Klebsiella and staphylococci were encountered comparatively seldom. The rate of the demonstration of Mycoplasma and viral bacterial associations varied depending on the epidemiological situation. In young persons, acute pneumonias were induced primarily by bacterial monocultures, Mycoplasma, and viral bacterial associations. The latter ones were demonstrated more frequently in persons over 60. The authors review the information content of the employed diagnostic program for the verification of bacterial monocultures and associations of causative agents. Note that penicillin, tetracyclines and biseptol are less effective in respect of bacterial associations detectable in elderly persons.