Absence of the fifth component of complement (C5) by immunochemical assay and marked deficiency by hemolytic assay (less than 0.1%) was found in a family in which the oldest male child had suffered severe and recurrent meningococcemia at age 15 years, two brothers developed meningococcal meningitis four years later (at ages 18 and 14 years), and a sister had the gonococcal arthritis-dermatitis syndrome.