Provided are monospecific and bispecific antibodies that are useful as anti-neoplastic agents and that bind specifically to human IGF-1R and human ErbB3. Exemplary antibodies inhibit signal transduction through either or both of IGF-1R and ErbB3. Exemplary polyvalent proteins comprise at least one anti-IGF-1R binding site and at least one anti-ErbB3 binding site. In certain embodiments the binding sites may be linked through an immunoglobulin constant region. AntiErbB3 and anti-IGF-1R antibodies (e.g., monoclonal antibodies) are also provided.
