Ninety patients with a history of recurrent upper and lower respiratory infections were randomized into three groups of 30 patients each. Group 1 was treated with i.m. immunoglobulins and oral polyvalent bacterial vaccine, group 2 with vaccine only, while group 3 was not submitted to prophylactic treatment. During and after prophylaxis, all three groups were evaluated for frequency of recurrent respiratory infections and the most relevant immunological parameters. In groups 1 and 2, a significant reduction of minor and major upper and lower respiratory infections was observed compared to the control group. Patients treated with Ig+vaccine or vaccine alone showed an increase of IgG2 subclasses and CD4 lymphocytes and positive changes of delayed skin tests. These findings confirm the results of previous preliminary studies which had shown the polyvalent bacterial vaccine to be useful for the reduction of recurrent infections of the respiratory tract, especially during the winter. Further studies will have to be carried out in order to identify the precise mechanism by which antigen stimulation with the oral vaccine improves the immunological response of the respiratory tract.