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Antitoxic immunity against diphtheria was surveyed during the period from 1962 to 1980. The survey was done by Schick test in the first decade and then by antitoxin titration of sera by the cell culture method in the recent six years. The data show clearly that the successful control of diphtheria has been accomplished in Japan as the result of active immunization. Both single (D) and combined (DP or DPT) vaccines were effective to convert Schick reaction negative. In addition, immunity was greatly improved by the introduction of the combined vaccine and one additional booster injection. However, such subsequent changes in the immunization schedule introduced in 1976 that giving the primary vaccination at older age and the omission of one booster injection at the preschool, has resulted in appearance of a high risk group below three years of age as well as in the lower immunity levels among school children. The data indicate that the continuous surveillance for diphtheria is required in this country.