The optimum conditions for carrying out the enzyme immunoassay (EIA) with a view to determine M. pneumoniae antigen and antibodies to them in the sera of patients with different respiratory diseases were established. The use of the specially modified EIA technique made it possible to reveal that patients with tuberculosis and chronic pneumonia showed similar occurrence of M. pneumoniae (35.7% and 35.0% of cases, respectively), while in patients with pulmonary sarcoidosis M. pneumoniae occurred in 27.2% of cases. At the same time the occurrence of antibodies in patients with chronic pneumonia and sarcoidosis was more than three times greater than in tuberculosis patients.