The currently recommended immunization scheme consists of diphtheria, tetanus and pertussis vaccine (DTP) given at the ages of 3, 4 1/2 and 6 months, followed by a reinforcing dose of the same vaccine at 18 months, and one of diphtheria and tetanus vaccine (DT) before school entry. Children injected according to this scheme may be over-immunized. The amount of diphtheria toxoid (25 Lf per dose) contained in DTP should not be reduced, because some of the children missing the third primary dose would be left unprotected until after the 18-month injection had been given. It is likely that the 18-month dose could be omitted without great danger if the preschool booster dose were assured, and more than likely that preschool boosting with DT is unnecessary if the full course, including the 18-month dose, has been given. Reinforcing doses of tetanus toxoid need not be given as often as every 10 years. They are probably not required except in the case of injury and may, even then, be omitted if less than 5 years have lapsed since the completion of basic immunization, or since the injection of the last booster dose. Non-adsorbed diphtheria and tetanus toxoids may be the preparations of choice whenever immunity has to be reinforced.