Provided herein are novel antiErbB3 and anti-IGF-IR antibodies (e.g., monoclonal antibodies). Also provided herein are novel monospecific and bispecific antibodies that are useful as anti neoplastic agents and that bind specifically to human IGF-IR and/or human ErbB3. Exemplary antibodies inhibit signal transduction through both or either of IGF-IR and ErbB3. Exemplary polyvalent proteins comprise at least one anti-IGF-IR binding site and at least one anti-ErbB3 binding site. In certain embodiments the binding sites may be linked through an immunoglobulin constant region.
