A specialized fluorometer was developed for measurement of fluorescence intensities of plant cell and protoplast suspensions after staining with fluorescein diacetate. With this apparatus, the suspensions in a cuvette can be mixed almost homogeneously at an optimal agitation speed (800 rpm). At this agitation speed, stable fluorescence intensities were obtained because there was neither cell and protoplast sedimentation nor destruction. For both Nicotiana tabacum BY2 and Catharanthus roseus L., linear relationships between the fluorescence intensities of cell/protoplast suspensions and viable cell/protoplast concentrations (from microscopic counting) were obtained. Viabilities of protoplasts calculated using fluorometry were proportional to their colony-forming efficiencies. With this apparatus, viable plant cell and protoplast concentrations can be measured with ease and accuracy.