IGF-1 receptor (IGF-1R) plays a key role in the development of numerous tumors. Blockade of IGF-1R axis using monoclonal antibodies constitutes an interesting approach to inhibit tumor growth. We have previously shown that h7C10, a humanized anti–IGF-1R Mab, exhibited potent antitumor activity in vivo. However, mechanisms of action of h7C10 are still unknown. Here, we showed that h7C10 inhibited IGF-1–induced IGF-1R phosphorylation in a dose-dependent manner. Also, h7C10 abolished IGF-1–induced activation of PI3K/AKT and MAPK pathways. Cell cycle progression and colony formation were affected in the presence of h7C10 probably because of the inhibition of IGF-1–induced cyclin D1 and E expression. In addition, we demonstrated that h7C10 induced a rapid IGF-1R internalization leading to an accumulation into cytoplasm resulting in receptor degradation. Using lysosome and proteasome inhibitors, we observed that the IGF-1R α- and β-chains could follow different degradation routes. Thus, we demonstrated that antitumoral properties of h7C10 are the result of IGF-1–induced cell signaling inhibition and down-regulation of IGF-1R level suggesting that h7C10 could be a candidate for therapeutic applications. © 2008 Wiley-Liss, Inc.