The purpose of this study was to investigate whether patients with chronic meningococcemia have abnormalities in their humoral immune system. The alternative and classical complement system, the levels of IgA, IgG and IgM, as well as IgG subclasses were studied in 15 individuals who had recovered from chronic meningococcemia. We found one individual with complete deficiency of properdin, a component of the alternative complement pathway. In the other patients, the complement system was normal. The mean plasma IgG concentration was significantly below normal in the patient group, while the mean values of IgA, IgM and the IgG subclasses were normal. Two individuals, however, had low IgG₂ and IgG₄ levels. We conclude that properdin deficiency and reduced plasma IgG levels may predispose to chronic meningococcal disease, but that the majority of patients with chronic meningococcemia have a normal humoral immune system.