An automatic method for viability assay of a population of cells is described. Cell populations are stained with trypan blue (for dead cells) and fluorescein diacetate (for live cells). A rapid cell spectrophotometer measures changes in light intensity which occur when individual cells pass through the photometric field. Tallies of the number of decreases (absorption and scatter by dead cells) and the number of increases (fluorescence by live cells) in light intensity are recorded and viability percentages are calculated. These automatically determined viability percentages are compared with viability percentages determined by visual counting and classifying of cells from the same populations.