In the microbiological examination of 132 children aged 0-3 years with acute bronchitis, Streptococcus pneumoniae and Haemophilus influenzae at a concentration of greater than or equal to 10(4) cells/ml, as well as different species of opportunistic bacteria, were isolated from tracheobronchial washings obtained from 100 of these children. S. pneumoniae and H. influenzae were found to play the leading role in the etiology of the acute bacterial inflammatory process in acute bronchitis in children.