The ability to develop delayed hypersensitivity was studied in 116 normal elderly subjects who were between 59 and 98 years old, with 2,4-dinitrochlorobenzene (DNCB) as an antigen. Only 84 (72%) were sensitized to the chemical. The impaired responsiveness was confined to those subjects more than 70 years of age, 69% of whom reacted, and was in contrast to the normal responsiveness (94%) of those 69 years of age or less. Sixteen subjects who reacted to tuberculin skin testing could not be sensitized to DNCB, suggesting that previously developed delayed hypersensitivity might persist despite the demonstration of anergy to a new allergen. Fourteen of 16 subjects with circulating antinuclear factor reacted to DNCB but only 3 of 6 with rheumatoid factor could be sensitized. No correlation was observed between serum immunoglobulin concentrations and reactivity to DNCB or tuberculin.