A method is proposed for the determination of in vitro plating efficiency as soon as cell attachment occurs. The estimate of plating efficiency is obtained by measuring the probability that no cells are attached within an area which is much larger than the area of a single cell and much smaller than the total area available for attachment. The technique is directly applicable to unirradiated control populations, while in irradiated populations its use is precluded by the similarity of attachment efficiencies of surviving and nonsurviving cells. The method may also be adapted to the determination of cellular multiplicity at various times after plating. [Chinese hamster lung V79 cells, Morris hepatoma viral transformed fetal mouse cells were used in this experiment.].