In approximately 25% of subjects receiving tetanus booster immunization (4 study groups A-D with n = 6, 25, 50 and 240, respectively; age 18-40 years for groups A-C, age 2 for group D), substantial IgE antibody levels were found. In groups A and B, sequential serum samples were available for analysis. The IgE response peaked before the IgG response and was often higher at week 1 than at week 2. The 6 subjects in group A (military recruits) received 2 booster injections. At the time of the second injection, 2 subjects had a very high IgE antibody titre (as well as a high IgG antibody titre), but no signs of an allergic reaction were noted. A significant association with atopy-related factors was found, but many apparently non-atopic subjects developed IgE antibodies.