The antibody response to polysaccharide vaccines is well known to be age variable, with younger infants or children responding less dependably and with lower antibody levels. The fate of these induced antibodies over time is less well understood. We studied the antibody kinetics of beta-Capsa 1, a Haemophilus B polysaccharide vaccine, in 185 children aged 17 to 71 months. Ninety percent of the children vaccinated at age 2 years or older achieved reportedly immune serum antibody levels three weeks after vaccination; 45% vaccinated at age 18 months achieved such levels. In six months, the antibody levels fell significantly in all children. In those vaccinated at age 18 months, the levels six months after vaccination were not significantly different from those prior to vaccination. Our study raises the possibility that antibody kinetics may be as critical a consideration as immediate antibody response in deciding the proper clinical use of a polysaccharide-based vaccine.