Thirty-seven healthy volunteers who received a pneumococcal polysaccharide vaccine were tested 4, 5, or 6 years after immunization for circulating type-specific pneumococcal antibody by radioimmunoassay of their sera. Each volunteer was immunized with one of four different pneumococcal vaccines containing 50 micrograms of each of 6, 8, 9, or 13 capsular polysaccharides; a few volunteers received octavalent or tridecavalent pneumococcal vaccines combined with bivalent influenza virus vaccine in a single syringe. Four years after immunization, the mean antibody level was 90% of the level achieved 4 weeks after vaccination. Among volunteers tested 5 years after immunization (including three 6 years after vaccination), the mean antibody level was 76% of that 4 weeks after inoculation. These findings confirm the long-term persistence of vaccine-induced type-specific pneumococcal antibodies and suggest that the interval between repeated doses of pneumococcal vaccine should be at least 5 years.