A murine monoclonal antibody with two antigen-recognition sites: one for CD3, an antigen expressed on mature T cells, and one for HER-2-neu, a tumor-associated antigen that promotes tumor growth. Ertumaxomab attaches to CD3-expressing T cells and HER-2-neu-expressing tumor cells, selectively cross-linking tumor and immunologic cells which results in the recruitment of cytotoxic T cells to the T cell/tumor cell aggregate.