In order to investigate the involvement of apoptosis in the pathogenesis of aplastic anaemia (AA) we measured the expression of the Fas receptor (membrane protein that triggers apoptosis), Fas ligand (FasL), bcl-2 (cytoplasmatic protein that blocks apoptosis) and p53 (nuclear protein that induces apoptosis) in CD3 and CD19 lymphocytes from the peripheral blood or bone marrow of controls, patients with AA, aplastic anaemia in complete remission (AA-CR) and multiply transfused patients without aplastic anaemia. The Fas receptor was overexpressed in both T and B lymphocytes from the peripheral blood and bone marrow from patients with AA. These abnormalities were not detected in AA-CR or multiply transfused patients. CD3/FasL cells were not increased and no FasL expression was detected in B lymphocytes. Bcl-2 was highly expressed in lymphocytes from controls, AA, AA-CR and multiply transfused patients (> 99% of positive cells) whereas p53 was not detected in any group. To further characterize the functional activity of the Fas receptor we performed a Fas-induced apoptosis assay in peripheral blood lymphocytes using an anti-Fas monoclonal antibody. The crosslinking of the Fas receptor transduced an increased apoptotic signal in lymphocytes from AA patients, but not in lymphocytes from controls, AA-CR patients or multiply transfused patients. Taken together, these data suggest that a Fas-based mediated apoptosis without the apparent participation of bcl-2 or p53 is a possible mechanism of lymphocyte depletion in patients with AA. In addition, these findings suggest that Fas expression is a continuous event occurring from progenitor bone marrow cells to mature cells.