The serum of a 29-year old woman with a recent episode of disseminated gonococcal infection and a history of meningococcal meningitis and arthritis as a child was found to lack serum hemolytic complement activity. The seventh component of complement (C7) was not detected by functional or immunochemical assays, whereas other components were normal by hemolytic and immunochemical assessment. Her fresh serum lacked complement-mediated bactericidal activity against Neisseria gonorrhoeae, but the addition of fresh normal serum or purified C7 restored bactericidal activity as well as hemolytic activity. The absence of functional C7 activity could not be accounted for on the basis of an inhibitor. Opsonization and generation of chemotactic activity functioned normally. Complete absence of C7 was also found in one sibling who had the clinical syndrome of meningococcal meningitis and arthritis as a child and in this sibling's clinically well eight-year-old son. HLA histocompatibility typing of the family members did not demonstrate evidence for genetic linkage of C7 deficiency with the major histocompatibility loci. This report represents the first cases of C7 deficiency associated with infectious complications and suggests that bactericidal activity may be important in host defense against bacteremic neisseria infections.