Immunity in meningococcal diseases has been inadequately studied and, until now, most attempts at serotherapy and immunization against meningococcal meningitis have been unsuccessful. To ascertain the status of immunity among children at greatest risk of contracting the disease, 267 serum specimens from healthy children aged from 1 month to 5 years were examined with Farr's radioimmunological technique, as simplified by Gotschlich. The results showed that the children must have received maternal antibodies through the transplacental route. Antibodies persisted up to the third month of life, after which they were virtually undetectable until, at about 8 months of age, antibodies reappeared. Subsequently there was a gradual rise, especially from 2 to 5 years, 97% of children in the highest age group having serum antibody levels of 0.479 mug/ml or more. These children may have had meningococcal infections that were wrongly diagnosed as respiratory disease of unknown etiology.