A flow-system technique has been developed that detects the high-frequency resistance and capacitance changes in a sensing orifice due to the passage of cells through the orifice. The resistance and capacitance changes are related to cell properties such as size, plasma membrane capacitance, and electrical resistivity of the cell interior. The relationship between measured impedances and cellular properties is discussed, and the prototype instrument for making such measurements is described. The instrument can simultaneously detect the dc Coulter volume and two ac parameters related to the complex ac impedance change. Some initial tests of the instrument using plastic microspheres and Chinese hamster ovary cells are described.