We used a limiting dilution method to estimate the proportion of T lymphocytes that could respond to PHA by producing IL-2, in peripheral blood mononuclear cells from healthy adult donors ranging in age from 18 to 81 years. The donors were selected using the guidelines of the SENIEUR protocol to exclude samples from donors not in optimal health. The frequency of PHA responsive, IL-2 producing T cells was found to decline with age, even though there was no corresponding change in the proportions of cells expressing the CD3 or CD4 determinants. There was, however, a statistically significant increase in the proportion of CD4 and CD8 cells expressing the CD45R0 determinant, thought to be a marker for memory T cells, and a corresponding decline in cells expressing the CD45RA marker found on naive peripheral T cells. The decline in the proportion of mitogen-reactive T cells in older donors, although statistically significant, was smaller than that seen in studies of aging mice, probably because the assay conditions for human T cell function are preferentially stimulatory for memory T cells, which accumulate in old age.