The authors present analysis of the data obtained in titration in the passive hemagglutination test of the sera obtained from 2498 persons with erythrocytes sensitized with specific polysaccharide of group A meningococcus. It appeared that in children aged under one year, even in those suffering from a generalized form of meningococcus infection, antibodies almost failed to be produced, whereas in older children the mean geometrical of the titre on the 5th-14th days of the disease constituted 1:343. The mean titres in the localized forms (nasopharyngitis and carriership) constituted 1:29.5 and 1:15.1, respectively; in the case of detection in the nasopharynx of meningococcus of group A the mean titre was 1:61. One to two months after sustaining any form of the infection the antibodies disappeared almost completely. Thus, the detected antibodies served as the indices of a recent meningococcus infection, including the asymptomatic one. From 12 to 49% of the persons with antibodies to meningococcus A in titres of 1:20 and over were found in the collective bodies in which meningitis patients were present. No antibodies were revealed in the donor sera collected in the localities with complete absence of meningitis, but in the localities with the recorded meningitis morbidity the sera contained antibodies to meningococcus of group A, the percentage varying from 1.5 to 40 depending on the epidemic curve level. A conclusion was drawn that detection in the collective bodies of over 10% of the persons reacting positively in the passive hemagglutination test with polysaccharide of group A meningococcus pointed to the threatening epidemic situation, despite the seeming welfare.