Fas/APO-1 (CD95) is a cell surface molecule that can transduce apoptotic signals into cells. We examined the expression of Fas antigen on CD4+ and CD8+ T cells of patients who received total body irradiation (TBI) as a preparative regimen for allogeneic bone marrow transplantation. Numbers of peripheral blood lymphocytes were significantly reduced after TBI. Cytofluorometric analysis revealed a significantly higher expression of Fas on CD4+ and CD8+ T cells after TBI. Serum soluble Fas concentrations were significantly elevated after TBI. Changes in the Fas system were therefore accompanied by TBI-induced lymphocytopenia, suggesting that Fas plays a role in irradiation-induced apoptosis in vivo.