Thymic function in myasthenic patients was examined using two biochemical markers which specifically define a population of cortisone-sensitive cortical thymocytes. The enzymatic activities of terminal deoxynucleotidyl transferase (TdT) and adenosine deaminase (ADA) were determined in 13 samples. High contents of both enzymes were found in young patients. The enzymatic activities were easily detectable also in the oldest patients, despite the morphological involution and the decrease in TdT which are known to occur with age in the normal thymus. TdT and ADA-containing cells were almost completely depleted in all the 3 treated patients by the corticosteroid treatment which provides a non-surgical alternative to the elimination of this lymphoid population by thymectomy. The persistence of TdT and ADA activity in old age, and their inhibition by the corticosteroid treatment.