The degree of the antilysozyme activity (ALA) of 430 Neisseria meningitidis strains isolated from 379 healthy carriers in 11 closed groups with different epidemiological situation and in 3 family foci was studied by the method of delayed antagonism. 87% of the strains exhibited ALA in a concentration of lysozyme between 1 and 10 micrograms per ml of the medium; about 11% of them were highly active, inactivating 20 and 25 micrograms of lysozyme per ml. ALA was shown to be the constant sign of carriership. The ALA of 39 strains isolated in cases of prolonged carriership (exceeding 2 weeks) was essentially higher than the ALA of 20 strains obtained in a single isolation. N. meningitidis strains of groups A, B and C, mostly responsible for the appearance of the generalized forms of diseases, were essentially less active than N. meningitidis strains of groups X, Y, Z, 29E, W-135, as well as strains not classified with any group. No correlation between ALA and the presence or absence of hyaluronidase was noted. Analysis of the data obtained in this study revealed that the ALA of N. meningitidis strains isolated in closed groups and families was not linked with the presence or absence of morbidity in the generalized forms of meningococcal infection and the spread of carriership in these groups, but reflected only differences in the serological picture of strains circulating there. The role of ALA in the formation of prolonged N. meningitidis carriership requires further study.