In aged as compared with young people T-cell immune responses tested by three different systems were significantly depressed; in the aged (more than sixty years) compared with the young (less than twenty-five) the number of positive delayed-type hypersensitivity reactions to five ubiquitous antigens was significantly lower; the lymphocyte response to the mitogen phyto-hæmagglutinin was significantly lower; and the presumed T-cell-dependent late IgG response to 5 μg. of monomeric flagellin was significantly lower. Furthermore, the mortality of very old people (over eighty), who were hyporesponsive in tests for delayed hypersensitivity, was significantly greater over a two-year period than that of comparable people who were not hyporesponsive.