Blood samples from 171 full-term pregnant women (aged 18-38 years) of middle socioeconomic status from Delhi were tested for diphtheria antitoxins by indirect hemagglutination (IHA) test. History of primary immunization/clinical diphtheria during childhood was not ascertainable, but none had been revaccinated against diphtheria at any time. About 94% women had very high antitoxin titers (> or = 0.125 IU/ ml); none had antitoxin titer less than 0.015 IU/ml, the minimum protective level. The titers were uniformly high in all age groups. However, women having 2 or more children had significantly higher antitoxin titers than those having no or one child (p < 0.01). The results from this study and historical data on diphtheria in Delhi are compatible with continued transmission of C. diphtheriae in recent times in Delhi which is of sufficient magnitude to boost the antitoxin levels in adults, especially mothers having two or more children. The study highlights the need of increasing the immunization coverage with DPT among children to reduce the transmission of Corynebacterium diphtheriae.