The immune structure of different professional groups with respect to L. pneumophila has been studied. A wide spread of Legionella infection among subway construction workers, subway personnel and railroad conductors has been established, which makes it possible to regard these professions as high risk groups susceptible to Legionella infection. The risk factors for this group are as follows: work in the atmosphere of high dust content with air supply by means of air conditioners, contact with soil in the process of earthwork. The bacteriological study of water samples obtained in dining cars has resulted in the isolation of two strains of L. pneumophila, serogroup I, which confirms the possibility of cases of Legionella infection among railroad personnel.