Tetanus and diphtheria antitoxin levels were measured in 183 urban adults, using a hemagglutination technique. Protective levels (>0.01 unit/mL) of tetanus antitoxin were present in more than 85% of sera from all subjects under 40 years of age and from men between 40 and 59 years old. A minority of women (36%) between 40 and 59 years of age, and of men (41%) and women (29%) over 60 years of age, were protected. Less than one quarter of the study group had protective levels (>0.01 unit/mL) of diphtheria antitoxin. The results of this study suggest that a minority of middle-aged women and older adults have been approximately immunized against diphtheria and tetanus.