In September 1982 two siblings were admitted to hospital within a few days of each other, with almost identical symptoms of meningococcemia. One of them had been discharged from hospital four days previously, fully treated to meningococcal meningitis. Two systemic meningococcal isolates and nasopharyngeal meningococci from patient No 1 were B:15:P1. 16 strains as well as one nasopharyngeal isolate from patient No 2. One nasopharyngeal isolate from the father was a non-encapsulated 15:P1. 16 strain. The two systemic isolates were clearly different with respect to the class 5 outer membrane protein(s); the second closely resembled the various nasopharyngeal isolates, all of which were identical. Only the two patients mounted detectable bactericidal antibody activity as measured by using human complement. Convalescent serum from patient No 1 after the second episode was bactericidal against the first but not the second isolate. No differences among patients and parents were found by measuring opsonizing activity. The clinical picture and the laboratory results seem to indicate that both children, one after a treated meningitis episode, had benign meningococcemia which subsequently ran its course untreated and without complications.