Immunoglobulin E and G levels to diphtheria and tetanus toxoids were investigated two years after a DT booster with either an adsorbed or a nonadsorbed, fluid vaccine, given at 10 years of age. Although IgE levels had declined, detectable IgE to diphtheria and tetanus toxoids were still found in 82% and 67% of samples, respectively, to be compared to prebooster levels of 3-14% and postbooster levels of 92-94%. The IgG levels had also declined but remained at equal and high levels in both the adsorbed and the fluid vaccine groups. The prevalence of allergic symptoms was similar in the two vaccination groups. Thus, the study showed an unexpected, long duration of the IgE responses elicited by a booster dose of DT vaccine. The booster dose also induced a durable, high IgG antibody response to both the adsorbed and the fluid vaccine.