In September 1981 a soldier died from meningococcal septicemia in a military camp in Mid-Norway. Soon afterwards one of his room-mates was transferred to a military camp in Northern-Norway where he shared sleeping quarters (room 7D) with 5 other soldiers of whom 2 fell ill with meningococcal disease 1 month later. Throat cultures were obtained from all 128 soldiers at the military camp in Northern-Norway; 41 (32%) harboured meningococci in their throats. The 3 invasive isolates and the isolates from the 4 healthy carriers at room 7D were all group B and type 15 meningococci. However, by DNA fingerprinting we could identify at least 2, probably 3, different individual strains among these 7 isolates. None of these strains were isolated from soldiers outside room 7D. By use of a B15 whole-bacterium ELISA method we showed that the levels of antimeningococcal IgG antibodies in the sera of the two cases at room 7D were low (18 and 28 OD units) compared with the mean IgG levels in the sera of their 4 healthy room mates (1150 OD units) and the mean IgG in the sera from all healthy soldiers (472 OD units).