Immunogenic properties of cytolysin were studied in experiments on guinea pigs. Preliminary immunization with cytolysin led to the suppression of response to ConA in lymphocytes not adhering to nylon wool and to the stimulation of response to Legionella antigens in lymphocytes adhering to nylon wool. For a month after infection with L. pneumophila the suppression of the proliferative activity of lymphocytes in the spleen of the immunized animals in response to ConA and Legionella antigens was observed, while in the lungs transitory suppression of response to ConA and Legionella antigens was followed by the restoration and then stimulation of proliferation in response to T-cell mitogen and specific antigens. The data obtained in these experiments indicate the capacity of cytolysin for modulating the development of immune response.