The functional expression of Fas-ligand on tumor cells reported in a variety of neoplasms has been proposed by several groups as a mechanism of tumor escape from immunological detection. To better support this hypothesis, we have evaluated and quantified for the first time the presence of the Fas(CD95)-R molecule on tumor-infiltrating lymphocytes and on matched peripheral blood lymphocytes (PBLs) of renal cell cancer patients. By two-color flow cytometry we have detected a significant increase in the Fas(CD95)-R expression on tumor-infiltrating lymphocytes compared with matched patient and normal volunteer PBLs. We also observed a decreased expression of the Fas(CD95)-R expression on PBLs from renal cell cancer patients compared with normal healthy controls. The Fas(CD95)-R expression was observed predominantly on the CD4+ subset in all three groups. These different distributions of the Fas(CD95)-R molecule support the hypothesis that the Fas(CD95)-R/Fas(CD95)-L pathway and tumor microenvironment play a major role in the modulation of T-cell function and differentiation to either memory and activation or apoptosis.