Single-chain bispecific antibodies are emerging as an extremely powerful and promising class of polyclonal T-cell-engaging proteins with hitherto unknown properties. Such bispecific antibodies have the potential to bypass most T-cell escape mechanisms selected in tumor cells during their development into solid tumors and metastases. With cytotoxic T-cells, bispecific antibodies can recruit the most potent effector cells for tumor cell elimination. Studying the mechanism of action of newly developed single-chain bispecific antibodies could provide insight into the shortcomings of bispecific antibodies that have been studied previously.