This study characterizes the early steps of T lymphocyte activation in elderly subjects. The expression of CD69, the earliest inducible antigen which appears with T lymphocyte activation, was assessed in T cells cultured with medium, anti-CD3 or PMA. The proliferative responses of T cells stimulated through CD69 and CD3 pathways were also studied. Donors included 31 healthy elderly [age mean (SD) 80(8) years] and 33 healthy young [age 30(5) years] subjects. In elderly people, the expression of CD69 was lower in T cells cultured with medium [3.4% (1.65-5.9; 25-75 percentiles) vs. 10% (6-18), p < 0.0003] and anti-CD3 activated [28.1% (16.5-53.8) vs. 79.5% (73-89), p < 0.0002] T cells. With PMA at 10 ng/ml, CD69 expression was higher in both groups of T cells, though still lower in the aged [84.5% (70.9-94.9) vs. 99% (65.7-100), p = 0.051]. CD69 T cells expression was equal in both groups with 2 ng/ml of PMA, but the co-stimulatory responses to CD69 under these conditions and in the presence of anti-CD3 were lower in the aged (16914 vs. 28904 cpm, p < 0.02) and (6944 vs. 14370 cpm, p < 0.02) respectively. Aged T cells failed to express CD25 at the same levels of young T cells when stimulated with CD69. These results suggests an age-associated defect in the very early steps of T lymphocyte activation that might influence later stages of lymphocyte function. An alteration in the transmission of the activation signal from the cell surface to protein kinase C may play a primary role in this defect.