Splenic T cells of C57BL/6 mice, ranging in age from newborn to 24 months old, were examined by two-colour flow cytometry using monoclonal antibodies (moAbs) to Thy-1, Lyt-1, Lyt-2 and L3T4. Thy-1+ cells in the spleen increased gradually after the birth, and reached a plateau around at 3 months of age, but did not show a significant age-related decline even at 24 months of age. Lyt subpopulations of T cells (Lyt-1+2+, Lyt-1+2-, Lyt-1-2+) showed a proportional increase until 1 month of age, but the onset of their imbalance was observed as early as 3 months of age. The percentage of L3T4+ subpopulation stayed at a relatively constant level throughout life (approx. 55%). At the individual cell level, Lyt-2 antigen was most vulnerable to aging and its membrane surface density showed a prominent decrease in 24-month-old T cells. An apparent decline was observed in the mitogen reactivity and cell-mediated cytolytic T lymphocyte (CTL) activity of old T cells or their subpopulations which were purified by the cell sorter.