Differential diagnosis of rheumatic carditis and nonrheumatic myocarditis in children is a challenging problem, which fact is connected with increase in those forms of rheumatic fever that present with few symptoms. The present article contains results of study into generally recognized routine methods of investigation common in the diagnosis of rheumatism. Measures of titers of antistreptolysine-O (ASL-O), antistreptohyaluronidase (ASH), antistreptokinase (ASK), biochemical indices for blood, activity of enzymes creatinphosphokinase, lactatdegidrogenase and its first fraction are not helpful; they do not permit distinguishing the above conditions, for which purpose we employed the method of fluid-phase immunoanalysis as recommended by V. N. Fedorich, A. E. Gaevskaia, A. I. Gritsiuk, I. D. Bul'da. The results obtained showed high informative value of measures associated with different antigens in A, G, M classes Immunoglobulins. Their alterations are to large extent dependent on the pattern of the pathologic process.