The activation state of thymic T and B lymphocytes was phenotypically and functionally explored in patients with Myasthenia Gravis (MG). We detected no phenotypic signs of activation in fresh total thymic lymphocyte suspensions (CD25 expression) while functional signs of activation were reflected by a significantly higher sensitivity to recombinant IL-2 (rIL-2) without any previous stimulation in MG patients as compared to controls. The response to rIL-2 was time-and dose-dependent, was inhibited by a blocking anti-IL-2 receptor antibody, and was associated to an increase of CD25+ T cells. Thymic B-cell populations purified after T cell and macrophage depletion, expressed at variable levels activation markers such as the transferrin receptor, the CD25, 4F2, CD23 and B8.7 Ag, indicating that a marked proportion of them are activated. Moreover, these B-cell populations were spontaneously sensitive to BCGF-12-kD and to a lesser extent to rIL-2, demonstrating that they also exhibit functional signs of activation. The largest proportion of activated B cells and the most intense response to BCGF-12-kD was found in patients presenting the highest anti-acetylcholine receptor (AChR) titers. Our data confirm the hyperactivity of the thymus gland in MG, reflected by the presence of T and B cells with functional signs of pre-activation. These cells could conceivably be located in lymphoid follicles and may represent autoreactive cells involved in the autoimmune process. Whether they are sensitized to AChR remains to be investigated.