Myeloid cells can mediate tumor cell cytotoxicity via certain receptors for immunoglobulins. Among the different Fc receptors, the high-affinity IgG receptor (Fc gamma RI, CD64) is a promising trigger molecule because it is selectively expressed on effector cells, including monocytes/macrophages and granulocyte colony-stimulating factor (G-CSF)-primed neutrophils. In vitro, a bispecific antibody (BsAb) (MDX-210, constructed by chemically cross-linking F(ab') fragments of monoclonal antibody (mAb) 520C9 to HER-2/neu and F(ab') fragments of mAb 22 to Fc gamma RI) mediated effective lysis of HER-2/neu overexpressing breast cancer cell lines. HER-2/neu (c-erbB2) is overexpressed in approximately 30% of breast and ovarian carcinomas and is a target for immunotherapy in clinical trials. In vitro assays showed Fc gamma RI-positive neutrophils to constitute a major effector cell population during G-CSF therapy. Based on these preclinical data and a preceding study at Dartmouth (New Hampshire) with a single dose of MDX-210 alone, a combination of G-CSF and MDX-210 is tested in a phase I study in breast cancer patients. In this study, patients receiving G-CSF are treated with escalating single doses of MDX-210. This therapy was generally well tolerated by the treated patients, some of whom reacted with fever and short periods of chills, which were temporally related to elevated plasma levels of IL-6 and TNF-alpha. After MDX-210 application, a transient decrease in the total white blood count and absolute neutrophil count (ANC) was observed. During G-CSF application, isolated neutrophils were highly cytotoxic in the presence of MDX-210 in vitro. These data indicate a potential role for G-CSF and BsAb in immunotherapy.