Fas-Fas ligand interactions play a central role in the regulation of the immune response. Fas ligand expression by tumors has been implicated in the abrogation of the host antitumor response by killing of Fas-positive effector lymphocytes. We have studied the presence and functional status of Fas and Fas ligand in Ewing's sarcoma. All Ewing's sarcoma cell lines tested expressed Fas on their surface. Three of the cell lines were readily killed after ligation of the Fas receptor. Four additional cell lines exhibited Fas-mediated apoptosis after preincubation with IFN-γ and/or cycloheximide, whereas two cell lines were resistant to Fas-mediated killing. With regard to Fas ligand, all cell lines examined were positive for protein by immunoblot, and specificity was confirmed by reverse transcription-PCR. However, using flow cytometric analysis, Fas ligand could only be detected in Ewing's sarcoma cells after permeabilization. Furthermore, the cell lines were not capable of inducing apoptosis of Fas-sensitive Jurkat cells. In addition, Ewing's sarcoma cell lines were able to serve as stimulators for the generation of cytotoxic effector lymphocytes and were susceptible to lysis by them. Therefore, Fas ligand is expressed in Ewing's sarcoma but is not functional, suggesting that Ewing's sarcoma is a potential target for immunotherapy.