Determination of cell viability is a procedure which is applicable to a number of techniques utilized in cell biology. Many means of determining cell viability have been proposed, each with its specific drawbacks. We present here two techniques for determining the viability of cells using an electronic particle analyzer, the Coulter counter. Viabilities of various tissue culture lines could be determined by comparing the ratios of live and dead cells obtained from a histogram of cells suspended in an isotonic solution or a solution of half-normal ammonium chloride (1/2 ACK). Viabilities obtained in this manner correlated well with those obtained by trypan blue dye exclusion. Viabilities of normal lymphocytes could be found in a similar manner by separating live and dead cells on histograms generated at high aperture currents. These values, though generally correlating well with dye exclusion data, may reflect determination of different criteria of cell death than previously used. The application of these techniques to a number of biological assays is discussed.