The bacteriological study of sputa, nasopharyngeal smears and bronchial washings taken from pneumonia patients has shown that the leading etiological agent was Streptococcus pneumoniae isolated in the diagnostic titre (10(7) bacteria per ml) in 78.1% of the cases. Staphylococcus aureus, Haemophilus influenzae, enterobacteria and yeast-like fungi have been found to play an insignificant role in the etiology of acute pneumonia (2.5 +/- +/- 0.9%). The results of the serological diagnosis by means of the complement fixation test have revealed that, alongside S. pneumoniae, the following infective agents are of etiological importance in cases of acute pneumonia: respiratory viruses (more than 50%), Mycoplasma pneumonia (10%), Chlamydia psittaci (6.4%) and Legionella pneumophila (3.8%). The study has first revealed that, under the conditions of Alma-Ata, serotypes 19, 23, 8 and 4 prevail among circulating pneumococci. This study has also shown that the use of M. pneumoniae antibody erythrocyte diagnosticum enhances the detection rate of mycoplasma infections in pneumonia patients.