Innovative treatment modalities are needed for Ewing's sarcoma (ES), a neoplasm with a disappointingly low survival rate despite the use of aggressive multimodal therapeutic approaches. We and others (D. Yee et al., J. Clin. Investig., 86: 1806-1814, 1990; K. Scotlandi et al., Cancer Res., 56: 4570-4574, 1996) have previously shown the existence and the pathogenetic relevance of an autocrine loop, mediated by the insulin-like growth factor-I receptor (IGF-IR), which is crucial for survival and proliferation of ES cells in vitro. Moreover, we reported that the IGF-IR-blocking monoclonal antibody (MAb), alphaIR3, as well as suramin, a drug that can interfere with growth factor by binding to the receptors, inhibited both the tumorigenic and the metastatic ability of ES cells in athymic mice. In this study, we analyzed whether agents that can block the IGF-IR-mediated loop are of value in association with conventional cytotoxic drugs for the design of more effective therapeutic regimens. Both alphaIR3 MAb and suramin treatment significantly increased the antitumor in vitro effects of doxorubicin and vincristine, two drugs with a leader action on ES. These findings were obtained by both simultaneous and sequential treatments. Analysis of the proliferation rate and of apoptosis revealed that alphaIR3 MAb and suramin significantly enhanced the G(1)-phase rate induced by doxorubicin, without substantially affecting doxorubicin-G(2)-M-blockage of cell cycle, and significantly increased the induction of apoptosis, which confirmed that the specific blockage of IGF-IR deprives ES cells of an important tool for the prevention of drug-induced apoptosis. Moreover, combination treatments of doxorubicin plus alphaIR3 MAb significantly increase the doxorubicin-induced impairment of the ability of ES cells to form colonies in soft agar. In conclusion, we showed that, in ES, the blockage of IGF-IR by a neutralizing MAb or by suramin may greatly potentiate the antitumor activity of conventional chemotherapeutic drugs.