GABRIEL, H., B. SCHMITT, A. URHAUSEN, and W. KINDERMANN. Increased CD45RA +CD45RO+ cells indicate activated T cells after endurance exercise. Med. Sci. Sports Exerc., Vol. 25, No. 12, pp. 1352–1357, 1993. Twelve endurance athletes performed either a 240-km cycle race in the Alps or an ultratriathlon (time: 731 ± 25 min). Two weeks before (S1) and on the 8th or 9th day after (S2) the competition, an exhaustive exercise at 110% of the individual anaerobic threshold was performed (stress test; S1: 21 ± 2 min; S2: 18 ± 2 min). Blood samples at rest, at 10 min, at end, and 60 min after S1 or S2 were taken for immunophenotyping of leukocytes with fluorescein- or phycoerythrin-conjugated monoclonal antibodies (CD3, CD4, CD8, CD14, CD16, CD45, CD45RO, CD45RA, CD69, HLA-DR) and dual color flow cytometry. Of the lymphocyte subpopula-tions, the CD45RO+-lymphocytes had higher cell numbers at S2 than at S1 (P < 0.01). This was due to the increase of CD45RA +CD45RO+ lymphocytes (P < 0.005), but not to the CD45RA +CD45RO+ lymphocytes (P > 0.05). Both the CD4+ (+39%) and CD8+ (+75%) cells were increased at S2. The activation markers CD69 and HLA-DR were not significantly increased at S2. Our conclusion is that a ∼12-h endurance exercise induces activation of T-cells, which is indicated by the increase of CD45RA +CD45RO+. This might also indicate an increase of memory cells by prolonged endurance exercise.