The Ewing family of tumors [EFT] (Ewing's sarcoma [ES] and peripheral neuroectodermal tumor [PNET]) are the second most common cancers in childhood and adolescence, and despite improvements in the treatment of these cancers with multi-agent chemotherapy, patients with recurrent disease, certain poor prognostic indicators, or suboptimal response to first line chemotherapy have few options for successful treatment of their disease and a low 5-year survival rate. Prior research established expression of the tyrosine kinase receptors c- kit and platelet-derived growth factor receptor-α (PDGFR-α) on EFT cell lines and the value of tyrosine kinase receptor inhibition in EFT with imatinib mesylate (Gleevec). The marine tunicate-derived agent ecteinascidin-743 (ET-743) has been found to be active in advanced, pretreated soft-tissue sarcoma and osteosarcoma in vitro and in vivo. This study sought to determine the therapeutic value of the combination of ET-743 and imatinib mesylate against EFT. Incubation with ET-743 for 4 hours in the ES cell lines, SK-ES-1, RD-ES, TC-71, and the PNET cell line, SK-N-MC showed cell death evident by light microscopy and reduced colony formation in clonogenic assays in all cell lines in a dose-dependent fashion. However, the combination of ET-743 and imatininib mesylate showed an antagonistic effect in the ES and PNET cell lines at all dose level combinations. These results suggest that ET-743 has cytoreductive anti-tumor activity in EFT, and warrants evaluation in clinical trials for patients with advanced ES and PNET. Further, caution should be exercised in evaluating the combination of ET-743 with a tyrosine kinase inhibitior in vivo. Additional investigation into the mechanism(s) underlying drug resistance encountered with this drug combination are warranted.