A bispecific fusion protein (H22-EGF) that binds simultaneously to the epidermal growth factor receptor (EGF-R) and to the high affinity receptor for the Fc portion of human IgG, Fc gammaRI (CD64), has been successfully constructed and expressed. For this construction, genomic DNA encoding the Fd fragment of humanized anti-Fc gammaRI mAb, H22, which binds Fc gammaRI at an epitope that is distinct from the Fc binding site, was fused to cDNA encoding human epidermal growth factor (EGF), a natural ligand for EGF-R. The resulting H22Fd-EGF-expressing vector was transfected into a myeloma cell line that was transfected previously with a vector containing DNA encoding the H22 kappa-light chain. SDS-PAGE analysis of purified H22-EGF demonstrated that the fusion protein was secreted predominantly as H22Fab'-EGF monomer (approximately 55 kDa), even though a free Cys residue exists in the hinge region of the H22 Fab' component. Using a novel bispecific flow cytometry-binding assay, we demonstrated that the purified bispecific fusion protein, H22-EGF, was able to bind simultaneously to soluble Fc gammaRI and EGF-R-expressing cells. H22-EGF inhibited the growth of EGF-R-overexpressing tumor cells and mediated dose-dependent cytotoxicity of these cells in the presence of Fc gammaRI-bearing cytotoxic effector cells. These results suggest that this fusion protein may have therapeutic utility for EGF-R-overexpressing malignancies.