An apparently healthy 18-year-old boy developed chronic bacteremia with Neisseria meningitidis, group C, without marked debility. Clinical manifestations included fever, skin rash, arthralgia, and headache. Serial immunoglobulin assays revealed the presence of a mild IgM deficiency and the absence of an IgG elevation in response to the infection. Titers of antimeningococcal antibodies to group C and group A meningococci, as determined by indirect immunofluorescence, revealed a disproportionately low IgM antibody response. The occurrence of this relatively rare form of meningococcal disease may be related to a mild immunological deficiency which is not detectable by routine laboratory tests and which is otherwise unassociated with an obvious susceptibility to infection.