The work discusses the results of studying by indirect immunofluorescence and the passive hemagglutination test antibodies to sacolemma antigens of myocardial muscle fiber in patients with rheumatic fever as compared to those in patients with affection of myocardium of nonrheumatic origin. The frequency of the detection of antibodies to sarcolemma antigens in sera of patients with rheumatic fever by immunofluorescence correlates with the activity of the process. No differences were revealed in the frequency of detection of antibodies reacting with sarcolemma in rheumatic fever and in other heart diseases in the active phase. Antibodies to sarcolemma antigens contained in HCL-extracts from human myocardium preparations, which are detected in the passive hemagglutination test are revealed significantly more often in sera of patients with rheumatic fever. Control experiments with IgG fractions obtained from whole sera were conducted to remove any possible nonspesific reactions of tissue extracts with the sera tested.