Three established Ewing's sarcoma (ES) cell lines (TC106, 6647, A4573), grown both in vitro and as xenograft tumors, were analyzed. In all 3 lines and tumors, the ES characteristic reciprocal translocation (11;22), as well as the presence of the ES-associated p30/32MIC2 antigen, were documented. However, these cell lines showed discrepancies in their neural and mesenchymal differentiation. The TC106 line was characterized by expression of the neuroendocrine marker secretogranin II (SgII) which was detectable by Northern blot and by radioimmunological detection (RIA) in the culture medium of secretoneurin, a proteolytic product of SgII. In contrast, TC106 cells were immunohistochemically and radioimmunologically secretoneurin-negative. Rapid cellular secretion of the peptide is probably the explanation of such a discrepancy. The 6647 and A4573 cell lines were SgII/secretoneurin-negative and the former presented morphological and immunocytochemical evidence of mesenchymal differentiation. In fact, the 6647 xenograft tumor showed osteosarcomatous-type morphological features and the neoplastic cells were immunocytochemically positive for osteonectin and osteocalcin antigens. Expression of the CgA gene, which is typical of neuroblastomas, was absent in the Ewing's sarcoma cell lines investigated. Our findings provide experimental evidence that Ewing's sarcoma is a heterogeneous tumor which may show either neuroectodermal or mesenchymal differentiation. SgII/secretoneurin analysis by hybridization and RIA procedures is a reliable approach to the identification of ES with neuroendocrine differentiation.