This disclosure provides a cell-based assay for testing the potency of multispecific binding molecules which specifically bind a T-cell antigen and a target antigen for redirected T-cell-mediated cellular cytotoxicity. The assay uses the TALL-104 T-cell line as effector cells, and provides a sensitive, specific, and reproducible method for ensuring that purity, activity, and stability of multispecific binding molecule batches can be measured for development, clinical trials, and commercial marketing.