To evaluate the immunogenicity of 23-valent pneumococcal polysaccharide vaccine in 52 nursing homes residents aged > or = 60 years, IgG antibodies to serotypes 1, 5, 6B, and 8 were measured by ELISA and compared before, and 1 and 12 months following vaccination. A significant immunological response for all serotypes was observed at 1 month after vaccination. The mean increase in antibody concentration was highly variable and ranged from 1.6 to 2.7. After 1 year, the mean concentrations remained significantly higher than prior to vaccination for serotypes 1, 6B, and 8, although there was a decrease in all mean IgG concentrations. Antibody levels were higher in men than in women, before and after immunisation. Post-vaccination values tended to be lower among subjects aged >75 years. Reduction in IgG concentrations by 33% 1 year after vaccination suggests that revaccination of institutionalised elderly people may be needed.