A patient is described who had a functional deficiency of the C8 complement component. His serum contained abnormal C8 which lacked some of the antigenic determinants of normal C8. The defect was associated with recurrent meningococcal infections due at different times to at least two strains. The patient's serum contained antibodies to meningococci and could induce phagocytosis and intracellular killing of the cocci by polymorphs. However, the serum was bactericidal only after the addition of C8-containing serum. As the patient did not give a history of susceptibility to other pyogenic organisms and has normal polymorph function, the circumstances of meningococcal infection must be unusual in that the plasma bactericidal activity critically determines the outcome: it may be that if large numbers of meningococci are not killed in the plasma, the polymorphs are overwhelmed.