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Proliferation of Chinese hamster cells B-14 was inhibited by irradiation, by incubation with 5-fluorouracil (5-FU) and by a combination of both treatments. The reduction in proliferation was assayed by the colony formation test, which was evaluated by an automatic colony analyser according to the number and volume of the colonies. It was demonstrated that the number of colonies multiplied by the volume was equivalent to the number of cells in a Petri dish and is called total colony volume. Since this quantity reflects the entire proliferation of cells, it is a more sensitive parameter for measuring cell viability than the clonogenicity of cells. The drug-radiation interaction showed a supra-additive effect, if total colony volume is taken into account, while the traditional scoring of colonies yielded only an additive effect.