Specific IgE and IgG antibodies were measured in 39 adolescent and adult humans preselected to have less than 0.125 antitoxin units (AU) ml to tetanus antitoxin (IgG) irrespective of previous immunization history. Mean antitoxin values (IgG) measured by sheep red cell hemagglutination (SRBC) increased by 2 logs to 1.49 AU/ml sera for tetanus and by 1 log to 0.025 AU/ml sera for diphtheria. Specific tetanus IgE was noted to increase after immunization from a mean 2.0 +/- 0.2% counts bound to 3.1 +/- 0.6% counts bound, but was not statistically significant. Diphtheria-specific IgE rose significantly (p less than 0.05) from a mean 1.5 +/- 0.2% counts bound to a mean 4.0 +/- 1.2% counts bound in atopic individuals and from a mean 1.2 +/- 0.0% counts bound to a mean 2.2 +/- 0.5% counts bound in nonatopics. These data document the development of tetanus and diphtheria IgE antibodies after booster immunization.