The immunocompetence of 22 subjects aged 85–104 years (mean 90 ± 1 years) was studied and compared to 21 young subjects aged 19–37 years (mean 30 ± 1 years). The absolute lymphocyte number and the percentage of T and B lymphocytes in the peripheral blood was similar in the two groups. A marked decrease in phytohemagglutinin response of T-enriched lymphocytes from old subjects was observed. Autologous mixed lymphocyte reaction (MLR) was also profoundly reduced in old subjects. No difference between male and female subjects was observed. The responsiveness of enriched T lymphocytes to allogeneic irradiated non-T cells was only slightly impaired in the old individuals. Non-T cells from old and young subjects functioned equally well as stimulatory cells in allogeneic MLR. The data suggest that an alteration of T lymphocytes with regulatory function and of self-recognition is present in aged h