An analysis of the signal (current or voltage change) generated by a particle passing the orifice of a Coulter counter is presented. Particular emphasis is given to the mathematical treatment of the signals generated by cells which are considered to be homogeneous in the interior. The theoretical approach comprises the analysis of signals induced by the cell (particle) at low electrical field strengths and also at higher field strengths where reversible breakdown of the cell membrane occurs. The calculations are based on the well-known model in which particles are treated either as spheres or ellipsoids located in the homogeneous electric field of a parallel plate condenser. General relationships are obtained between the Coulter counter signals induced by a particle and its intrinsic parameters (e.g. internal conductance and membrane resistance). On the basis of these relationships information about the internal conductance of a cell can be obtained when the membrane potential is taken to the breakdown value. Furthermore, it can be shown that some conclusions about the geometry of the breakdown in the cell membrane can be drawn from the analysis of the Coulter counter signals. The calculations presented here also allow one to derive equations for the membrane potential which are of particular relevance in breakdown experiments.