Clinical studies of 6- and 12-valent pneumococcal capsular polysaccharide vaccines were carried out in controlled studies among novice gold miners in South Africa. In the studies 1,523 persons received pneumococcal vaccine, and 3,171 were included as controls. In the great majority of subjects given pneumococcal vaccine, antibodies developed against the capsular types included in the vaccine. The 6-valent vaccine afforded 76% reduction in cases of laboratory-verified pneumococcal pneumonia caused by the homologous types, and there was 92% reduction in the cases afforded by the 12-valent vaccine. The vaccines were well tolerated and offer great promise for effective control of illnesses caused by pneumococci.