The inactivated poliovirus vaccine is heat stabile, gives high serum IgG concentrations but less pronounced mucosal immunity and must be given as repeated injections. A new enhanced-potency Dutch inactivated vaccine could circumvent these difficulties. We compared antibody concentrations measured as neutralization or ELISA titers, and avidity of serum and salivary antibodies in children vaccinated with three doses of the earlier Swedish vaccine given over nine months or the new antigen-rich vaccine. After three doses, but not after two, serum neutralization titers for type 1 and type 3 poliovirus were higher using the new vaccine but secretory IgA levels in saliva were similar. The avidity of the serum IgG antibodies was significantly higher after two doses of the new vaccine than after three doses of the old. Thus the new vaccine gives excellent antibody responses of high titers and avidities, but should preferably be given in three doses.