The histogenesis of Ewing's sarcoma (ES), the second most frequent primary bone tumor in humans, remains controversial. A new cell line (SIM-1) was derived from a peripheral neuroectodermal tumor (PNET) and used for the production of a monoclonal antibody (HBA-71), which recognizes a novel cell surface antigen of ES- and PNET-derived cells and paraffin-embedded tumor sections. The HBA-71 antigen expression is restricted to PNET/ES and the antigen was not detected on cell lines or tissue sections of any other tumor tested, with the exception of ependymoma. Three proteins with molecular weights of 300,000, 185,000, and 90,000 were isolated from SIM-1 membrane extracts by HBA-71 affinity chromatography. Trypsin treatment of intact SIM-1 cells destroys the HBA-71 epitope and cleaves off two proteins with molecular weights of 210,000 and 95,000. HBA-71 antigen expression is not influenced by treatment of ES cell lines with differentiation inducers. Within normal tissues reactivity was observed with the adenohypophysis, ependymal cells, endocrine pancreas, Sertoli, and ovary granulosa cells. The reagent links ES with PNET and provides a highly valuable probe for (a) the immunohistological differential diagnosis of ES/PNET using fresh tissue or paraffin sections from other small round cell tumors, (b) the histogenetic studies of ES/PNET, and (c) the in vivo diagnostic and therapeutic procedures in patients with ES and PNET.