The present invention includes a method of measuring cytolytic activity including providing a device capable of monitoring cell-substrate impedance operably connected to an impedance analyzer, adding target cells to at least one well of the device, adding effector cells to the at least one well, monitoring impedance of the at least one well and optionally determining a cell index from the impedance, wherein monitoring impedance includes measuring impedance during at least one time point before and at least one time point after adding effector cells, and determining viability of said target cells after adding effector cells by comparing the impedance or optionally the cell index at the at least one time point after adding effector cells to the impedance or optionally the cell index at the at least one time point before adding effector cells.