Abstract: The present invention provides a method of determining the cytotoxic effect of an effector molecule or cell on a target cell presenting a heterologous peptide comprising: (a) incubating said effector molecule or cell with said target cell, wherein said target cell is immobilised on a substrate comprising at least one pair of electrodes; and (b) determining the impedance during and/or after step a) wherein a decrease in impedance is indicative of target cell death.