In a study of the issue of revaccination with Haemophilus influenzae b polysaccharide vaccine, 36 children who had been vaccinated with polysaccharide vaccine at age 18 to 20 months were stratified according to their 3-week postvaccination anticapsular antibody concentration determined by radioantigen binding assay, then sequentially assigned to one of three groups so as to have comparable distributions of peak antibody concentrations. These children were revaccinated with polysaccharide vaccine at 24 to 27, 30 to 33, or 36 to 39 months of age. The post-reimmunization geometric mean antibody concentrations in these three groups were 3.1, 3.0 and 7.8 micrograms/ml, respectively, and the percentages rising to greater than 1 microgram/ml were 75, 80 and 93%. The geometric means and antibody response rates were nearly identical to age-matched control groups not previously vaccinated. Thus there was no evidence of priming (memory induction) or of tolerance induction by the early (18 months old) primary vaccination.