A simple, reproducible assay which quantitatively expresses viable cell number in monolayer cultures is described. The assay is based on the constancy of total intracellular potassium per unit cell number under a wide variety of experimental conditions. Intracellular potassium is measured by the exchange with extracellular 86Rb upon attaining isotopic equilibrium. The Rb exchange by 106 viable cells is not changed during logarithmic growth or during antibody-complement mediated lysis. Similarly, cytotoxins from activated lymphocytes damage cells without altering Rb exchange. Agents which alter Rb influx (ouabain) or cause unbalanced growth (vincristine, mitomycin) alter the Rb exchange per 106 cells, and under these conditions, Rb uptake does not reflect viable cell number. This method promises wide application in assays utilizing target cells grown as monolayers.