Bacteriological confirmation of the presence of group A Streptococcus (GAS) in patients with acute rheumatic fever (ARF) by means of the express method was achieved only in 15.1% of the cases and the positive growth of the culture was observed only in 12.1%. Streptococci isolated from the patients showed a low IgG-Fc receptor activity just as in the negative control. The blood serum of patients exhibited the presence of antibodies to 18 OF-serotypes among which serotypes 2, 4, 22, 28, 48, 68, 75, 77, 78, PT 2841 prevailed; their content fluctuated from 12.5 to 48.4%. There were no differences between the nature of distribution of the given antibodies depending on the presence and level of ASL-O in the blood and the presence of concomitant chronic tonsillitis. Antibodies to IgG in titres 1:10-1:40 were revealed in the blood of only 18.1% of the patients; there was some dependence of the frequency of their detection on the markedness of carditis and the presence of cardiac defects.