The results of serological surveys, carried out with a view to the detection of capsular meningococcal polysaccharides in the passive hemagglutination (PHA) test and covering 1,289 patients with systemic meningococcal infection, 221 patients with bacteriologically confirmed meningococcal nasopharyngitis, 2,820 persons in organized groups of children and adults with different epidemiological situation (including 650 carriers) and 4,050 residents of 8 cities with different morbidity levels, were analyzed. The patients, the carriers and the members of organized groups underwent multiple surveys. As shown in this study, the confirmation of the meningococcal etiology of the systemic forms of meningococcal infection (meningitis, meningococcemia) in all cases with the exception of children under 1 year of age, as well as the determination of the groups of meningococci, could be achieved in the PHA test twice as frequently, i.e. in 60-70% of cases, as with the use of the bacteriological method. The same regularities were observed in nasopharyngitis. The conclusion was made on the possibility of using the PHA test for more exact determination of the group of meningococci, prevailing at the initial stage of diseases, this determination playing an essential role in the epidemiological surveillance. The data obtained as the result of serological surveys in organized groups may lead to conclusions on the circulation of meningococci and their group composition, thus showing the necessity of profound epidemiological study with ensuring epidemic-control measures. The selective serological study of some hundreds of donor sera is not expedient in big cities, as it provides no data on the circulation of meningococci there.(ABSTRACT TRUNCATED AT 250 WORDS)