The prophylactic action of polycomponent vaccine B[symbol: see text]-4, prepared from the antigens of opportunistic bacteria, on morbidity rate in acute respiratory diseases (ARD) of bacterial and mixed (bacterial and viral) etiology in 121 children aged 2-5 years, attending pre-school institutions was evaluated. For comparison, a group of 118 children of the same age from the same institutions was formed. The vaccine was introduced after the schedule consisting of 3 intranasal and 6-9 oral administrations made at intervals of 3-4 days. The duration of the course of immunization was 26 +/- 4 days. The prophylactic effect of B[symbol: see text]-4 on ARD morbidity was evaluated by the number of ARD cases and their duration per child. The prophylactic effect of B[symbol: see text]-4 on ARD morbidity lasted 14 months (the term of observation) after immunization and was manifested by a decrease in the number and duration of ARD cases after administration of the preparation, also in a group of highly susceptible children.