The functional status of T cells was examined with regard to age in normal humans. T-Cell cytotoxicity (CMC) as determined by a xenogeneic assay in vitro was found to be depressed (P < 0.02, Wilcoxon rank test) in the oldest group (Group 3: 60-70 years) compared to the two younger groups (Group 2: 50-59 years; Group 1: <50 years). CMC activity of Groups 1 and 2 did not differ significantly (P > 0.2). Both the optimal (P < 0.02) and suboptimal (P < 0.001) PHA responses were depressed in Group 3 compared to Groups 1 or 2. There was no significant difference between Concanavalin A responses or percentage B and percentage T cells among the three groups studied. These results are consistent with a selective decline in some T-cell functions with age in a human population.