A cross-sectional survey examined whether habitual endurance exercisers retained a higher level of T cell function than sedentary individuals in old age. Subjects, all male, comprised 17 elderly runners, 16 young, and 19 elderly controls (mean ages +/- SD: 63.8 +/- 3.3, 23.6 +/- 1.6, and 65.8 +/- 3.5 yr, respectively), whose resting blood samples served for the immunological tests. Compared with the young subjects, both elderly groups had lower circulating CD3+ and CD8+ cell-counts (P = 0.029, P = 0.001, respectively), with a trend to a higher CD4/CD8 ratio, but higher percentages of activated CD3+, and "memory" CD4+ and CD8+ cells (all, P < 0.0001). Proliferative responses to phytohemagglutinin, pokeweed mitogen, and alloantigens were markedly reduced in the elderly (P < 0.001, and P = 0.024, respectively). IL-2 production tended to be decreased in the elderly sedentary subjects. However, natural killer cell activity and other cytokine production remained unchanged in the elderly sedentary subjects. Comparison between the active and sedentary elderly groups showed no differences in circulating counts of immunocompetent cells. However, the active elderly subjects demonstrated significantly greater proliferative responses to phytohemagglutinin ( P = 0.016) and to pokeweed mitogen (P = 0.011), and higher rates of IL-2 (P = 0.021), IFN-gamma (P = 0.015) and IL-4 production (P = 0.012). These results suggest that endurance training in later life is associated with a lesser age-related decline in certain aspects of circulating T cell function and related cytokine production.