We vaccinated 15 persons, age 56 to 79 years, with 14-valent pneumococcal polysaccharide vaccine and revaccinated them with 23-valent vaccine 6 years later to assess adverse reactions and booster responses of anticapsular antibodies. No systemic reactions occurred after administration of either vaccine. After booster vaccination, five vaccinees developed only mild soreness or tenderness at the site of injection. The (arithmetic) mean antibody level to 12 capsular types measured by radioimmunoassay increased 3.1-fold 1 month after the first vaccine; the range was 1427-5124 ng antibody nitrogen per ml (N ml-1). Six years later, mean antibody levels waned to about half these levels. One month after administration of the second dose of vaccine, the mean antibody level increased 1.5-fold; the range was 1011-3954 ng antibody N ml-1. Revaccination with pneumococcal vaccine of elderly persons can be carried out safely; the levels of anticapsular antibody achieved after revaccination are about half the levels after primary vaccination.