Measurement of electrical impedance is a relatively new real-time and label-free method for monitoring cell adhesive properties. Impedance measurements are performed in tissue culture wells in which the bottom is equipped with gold electrodes. The extent of electrode coverage by living cells as well as the strength of the bond between the cell membrane and the electrode surface determines the impedance, which in real-time cell electrical sensing (RT-CES, ACEA Biosciences, San Diego, CA) is measured as the cell index (CI). We showed for carcinoma cells that CI was linearly correlated to the number of cells and that CI also was related to the amount of coating (laminin-5) of the wells. When natural killer (NK) cells were added to adherent carcinoma cells (target cells) CI declined rapidly dependent on the NK cell:target cell ratio. The initial decrease of CI was much more pronounced than target cell death as measured by [(3)H]thymidine incorporation assay. Such a rapid fall of CI was due to changes in the adhesion and morphology of target cell undergoing apoptosis. It took more than 6 h before the extent of cell death and fall of CI were comparable. We also showed using A431 cells and an antibody specific for the human epidermal growth factor receptor (Erbitux, manufactured by Merck KGaA, Darmstadt, Germany) that RT-CES could be used to monitor antibody-dependent cellular cytotoxicity. Thus RT-CES is a convenient way to continuously determine cell number and cell adhesion and may offer early detection of NK cell-mediated cytotoxic effects.