A healthy 5½ year old girl developed Neisseria meningitidis, group Y meningitis and bacteremia; the organism was sensitive to the antibiotics employed. Ten months after full recovery a second episode of group Y N. meningitidis meninigitis occurred. Appropriate leukocytosis and neutrophilla were present. Serum immunoglobulin levels were normal, as were antibody titers to blood group and typhoid antigens. Class specific fluorescent antibody titers to autologous meningococci, bactericidal antibody, and antibody to meningococcal protein antigen were elevated. There was a complete lack of serum hemolytic complement activity due to absence of the sixth component (C6). C3 was transiently depressed in association with a disseminated coagulopathy; C8 levels were 1/3 of normal; the remaining components were comparable to reference sera. Complement mediated opsonization of zymosan was normal, as was polymorphonuclear leukocyte killing of opsonized S. aureus and K. coli. The early acting components of the alternative complement pathway were functionally intact. T and B lymphocyte numbers and response to mitogens was normal. No coagulation defects were demonstrable. The patient's mother and uncle had half normal levels of C6. The two previously reported cases of C6 deficiency had recurrent N. gonorrhea and meningitidis infections respectively, underscoring the significance of this late complement component in host defense against Neisseriaspecies. (Grants fiD 00413, At 07051 and AI 10286)