The influence of the content of bound sialic acids in mucin on its capacity to decrease the infective dose of meningococcal culture has been studied; as a result, the direct relationship with a high degree of correlation between these two characteristics has been revealed. The direct relationship between the content of bound sialic acids and the viscosity of aqueous mucin solutions has also been revealed in this study. The above-mentioned biological activity of mucin is not observed in the presence of free sialic acids.