Bispecific antibodies were constructed using genetically encoded unnatural amino acids with orthogonal chemical reactivity. A two-step process afforded homogeneous products in excellent yield. Using this approach, we synthesized an anti-HER2/anti-CD3 bispecific antibody, which efficiently cross-linked HER2+ cells and CD3+ cells. In vitro effector-cell mediated cytotoxicity was observed at picomolar concentrations.