Antibodies against Neisseria meningitidis in sera of childrens suffering from cerebrospinal fever and in sera of their family contacts were assayed by serum bactericidal test, indirect immunofluorescence and passive hemagglutination. Changes in antibody titers were showed in the course of the observations in many patients but only in few contacts. The immune response during meningococcal disease and the sensitivity of different serological methods were discussed.