Nasopharyngeal swabs for the isolation and identification of meningococci, and blood specimens for the detection of antimeningococcal A, B and C antibodies were obtained from 341 Army and Navy recruits in the spring and autumn 1977. Samples were taken twice, at the arrival at the training camp and after 25 days. The main findings are as follows. 1) About 1/3 of the inductees at the arrival at the training camp were carriers of meningococci. Strains of serogroup B were more prevalent, followed by non-typable strains and strains of group C. After 25 days of residence in the camp the prevalence of carriers of meningococci has increased by 42%, and about half of the recruits were carriers. The prevalence of serogroup C has increased by 86%, of the non-typable strains by 48% and of serogroup B by 36%. 2) At the induction, 73% of the recruits had anti-B antibodies, whereas the corresponding figure was 33% for anti-A and 24% for anti-C. After 25 days in the camp the prevalence of anti-B antibodies was 83%, of anti-A 36% and of anti-C 47%. 3). At the induction the prevalence of meningococci B was higher among those who had no anti-B antibodies. Furthermore, the probability of a recruit who was not carrier of meningococcus B at the induction to become a carrier of meningococcus B at the second sampling was higher when he had no anti-B antibodies at the induction. Similar associations were not evident for serogroups A and C probably on account of the much smaller numbers in these groups. 4) The carrier state is associated with an increase of the titre of the corresponding antibodies.