Indirect haemagglutination test for antitoxic diphtheria antibodies was performed on 3503 sera of persons of all age groups. The results demonstrate that a high percentage of juveniles and adults with less than 0.01 IU/ml has either no (48.9% for juveniles, 35.6% for adults) or at 0.01-0.09 IU/ml only insufficient immunological protection (30% and 41%, respectively) against diphtheria and its toxic effects. But even among children up to 14 years of age, 28.5% are completely without and 20.5% without reliably protective antitoxic immunity, despite a high rate of immunization. There is thus a dangerously high epidemic potential of susceptible persons in the population of the Federal Republic of Germany. To erect a barrier of immunity against epidemic spread requires not only immunization of all children, but also re-immunization every five to six years, plus immunization of juveniles and young adults among a selected group with, at present, highest risk of infection. For this purpose there is a diphtheria toxoid of Behringwerke for adults with only 5 IU toxoid/0.5 ml, which, however, after single application produced an antitoxic immunity (greater than or equal to 0.1 IU/ml) in previously sensitized subjects (17 of 49). Of 32 primary seronegative persons 14 reacted to two injections, while a further 18 (36.7%) had no immune response within six weeks even after second injection.