An outbreak of meningitis in Royal Air Force recruits due to Neisseria meningitidis Group C type PI, 2 gave the first opportunity for polysaccharide vaccine to be used for controlling such an outbreak in the U.K. The effect of the vaccine on an epidemic in a large recruit training centre was studied after chemoprophylaxis had failed. With the possible exception of one vaccinee, further cases did not arise among camp personnel. Vaccination was continued until the carriage rate of the epidemic strain among recruits leaving the camp had fallen from greater than 19 to less than 1%. Two persons who had only indirect contact with the camp developed the disease. Vaccination early in the course of such an outbreak appears to be a useful and practical method of limiting symptomatic infection but not acquisition of the epidemic strain.