This study was performed in order to investigate the type of T cells recruited to the blood in response to an acute bout of exercise with regard to mean lengths of telomeric terminal restriction fragments (TRF) and surface activation markers and with special emphasis on age-associated differences. Ten elderly and ten young humans performed maximal bicycle exercise. There was no difference in the number of recruited CD4+ and CD8+ cells between the young and elderly group. In both age groups the immediate increases could be ascribed to recruitment of CD28- cells (CD8+ and CD4+ cells) and memory cells (only CD8+ cells). Furthermore, after exercise mean TRF lengths were significantly reduced in blood mononuclear cells and in CD8+ cells from young subjects and in CD4+ cells from elderly subjects compared with lengths pre-exercise. These findings suggest that the mobilization of T lymphocytes during acute exercise is mainly a redistribution of previously activated cells with an increased replicative story than cells isolated from the blood at rest. Furthermore, elderly humans fulfilling the Senieur protocol have a preserved ability to recruit T lymphocytes in response to acute physical stress.