A novel humanised monoclonal antibody (Mab, h7C10) was raised against the human insulin-like growth factor-1 receptor (IGF-1R); it exhibited potent inhibition of tumour growth in animal models. Further evaluation of its inhibitory activity at hybrid receptors (Hybrid-Rs) composed of the association between IGF-1R and insulin receptor (IR) was performed. Selective, potent and efficacious inhibition of [(125)I]IGF-1 binding as well as IGF-1- and IGF-2-mediated receptor phosphorylation was demonstrated at both IGF-1R and Hybrid-Rs, without activity at IR. Ligand-independent down-regulation of both IGF-1R and Hybrid-Rs was obtained upon long-term association with h7C10. In vivo evaluation was performed in a MDA-MB-231 xenograft mouse model, showing a 14-fold higher level of Hybrid-Rs as compared to IGF-1R. A more potent anti-tumoural response was obtained for h7C10 as compared to Mabs targeting solely IGF-1R or Hybrid-Rs. The herewith described neutralising properties of h7C10 as potent inhibitor of both IGF-1R and Hybrid-Rs are likely to participate in its anti-tumoural activities and maybe of interest for therapeutic applications.