We have characterized and evaluated the antitumor properties of trastuzumab-IFN-β mutein in HER2-expressing cancers. Since IFN-β activates antitumor immune responses, it is expected to be administered in combination with immunotherapeutic drugs. Therefore, the study suggests that trastuzumab-IFN-β mutein is a promising candidate for the treatment of HER2-positive carcinoma.