A possibility of using the passive hemagglutination test with meningococcus diagnostic agents of groups A and C for observing the changes in the infectious process was demonstrated. Sera of patients with generalized form of meningococcus infection showed an increase (by the 2nd--3rd week) and a rapid reduction (by the 4th week) of the specific antibodies titres to the levels seen during the first days of the disease. At the remote periods after the disease--in 2 to 5 years--hemagglutinins were revealed in the same titres as in healthy persons (1:10--1:13). Study of physico-chemical nature of antibodies demonstrated that at the early stages of meningococcus infection there formed antibodies referred chiefly to macroglobulins, and at the remote periods (after 3 months)--to microglobulins.