Diphtheria and tetanus antibodies were determined in a random sample of 635 test persons of 1 to 25 years of age via the passive haemagglutination test. Using sera and immunoglobulins standardized for protective capacity, titres were expressed as international units (IU). Titres greater than or equal to 0.01 IU/ml diphtheria antitoxin were detected in 96% of children (1-17 years, n = 70), in 79% of adults aged 18-22 (n = 397), in 68% of adults aged 23-25 (n = 168); tetanus antitoxin was found in 96% of children and 90% of adults. 9% of the males between 18 and 22 years of age had no detectable tetanus antibodies before of the same age had no tetanus antibodies. Safe protection (greater than or equal to 0.1 IU/ml) against diphtheria may be expected in 56% of children and in 35% of adults only, against tetanus in 86% and 75%, respectively. 25% of all test persons had excessive tetanus titres (greater than or equal to 10 IU/ml).