A serological study to evaluate the efficacy of the diphtheria immunisation programme in Singapore based on sera collected from 425 healthy persons between 6 months and 40+ years of age showed that 94.6% of the population surveyed possessed protective neutralising antitoxin (greater than or equal to 0.01 IU/ml). No sex or ethnic difference was noted. The most suspectible age-group was adults above 40 years of age in which 14.3% had no protective antitoxic immunity against diphtheria (less than 0.01 IU/ml). The geometric mean titre was generally high with sharp anamnestic type of secondary response elicited by periodic boosters administered. The survey confirmed the efficacy of the childhood immunisation programme which extended from 3 months to 15+ years of age, in maintaining a high level of herd immunity of the population right into adulthood. Further extension of the programme to include routine revaccination of adults was not considered necessary.