Apoptosis mediated by the interaction of cytotoxic T lymphocyte with blast cell via Fas receptor/Fas ligand (FasL) pathway is a one of the mechanisms of immunological leukemia surveillance. There is few data on possible blocking of Fas receptor by soluble form of Fas (sFasL) present in serum and the role of blast cells as the source of this ligand. Forty-eight patients with de novo diagnosis of acute leukemia, 32 with myeloblastic (AML) and 16 with acute lymphoblastic leukemia (ALL) were studied. Fas expression on bone marrow leukemic blasts was determined by flow cytomertic immunofluorescent analysis and serum concentration of sFasL assessed at presentation, in remission and in relapse. Blasts of all patients expressed Fas at variable degree (0.8 to 100%). Fas expression was significantly higher on myelo--than on lymphoblasts. There was no relation between degree of Fas expression at diagnosis and remission rate. Concentration of sFasL in acute leukemia patients at diagnosis was significantly higher than in healthy control group, decreased to normal values in remission and rose again in relapse. There was a negative correlation between Fas expression on blasts and sFasL concentration at the time of diagnosis. Results obtained suggest that blast cells could be the source of soluble FasL in acute leukemia patients and that sFasL serum concentration could be used for monitoring of disease activity.