Disclosed are methods and compositions for inhibiting the growth of a tumor (e.g., a malignant tumor) in a subject. In particular, combination therapies for treating a tumor in a subject by co-administering either i) an effective amount of an anti-estrogen agent or ii) an effective amount of a receptor tyrosine kinase inhibitor and an effective amount of a bispecific anti-ErbB2/anti-ErbB3 antibody, and optionally an effective amount trastuzumab. Also disclosed is a bispecific anti-ErbB2/anti-ErbB3 antibody for use in the therapy of a tumor in combination with either i) an anti-estrogen agent or ii) a receptor tyrosine kinase inhibitor, and optionally in use with trastuzumab.
