The decays of 64Cu incorporated in human malignant (A549) or monkey nonmalignant (CVI) cells lead to cell death. When plotted as a function of the radioactivity introduced in the growth medium (microCi/ml at t = 0), the residual colony-forming capability decreases exponentially. The slope of the corresponding curve is steeper for A549 than for CV1 cells. Different data show that the cellular lethal event is a consequence of 64Cu transmutation and not of the irradiation by the simultaneously emitted beta- and beta+ particles. Liquid holding results show that the lethal event is irreparable. The decays of 67Cu, another radioisotope of copper, lead to cell death with the same exponential survival curve and the same lethal efficiency as for 64Cu, in spite of their different decay schemes. The lethal efficiency of both copper isotopes is close to that of 125I utilized in the form of iododeoxyuridine under the same experimental conditions as 64Cu and 67Cu. The high lethal efficiency of radioactive copper transmutations raises questions about the role in DNA functioning of copper atoms known to be present in trace amounts in this macromolecule. The lethal consequence of radioactive copper transmutations suggests that the copper atoms bound to DNA are essential for cellular functioning.

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Summary. Be G 1 the value of G corresponding to thymidine irradiated with X-rays at 0 ~ in aqueous solution; be G" its value for thymidine irradiated under other experimental conditions. Be D 1 the value of X-rays doses which leaves a 37% survival of bacteria or transforming DNA irradiated at 0 ~ in saline; be D' its value under the same other conditions. The loose dependance of the bacterial sensitivity to the modifications of the surroundings seems to be a proper~y of the cells. The part played by decreasing of temperature seems due to the DNA of the bacteria.l~gsumd. Soit G 1 la valeur de G correspondant s la thymidine irradi4e par rayons X 0 ~ en solution aqueuse; soit G' sa valour correspondant ~ la thymidine irradi6e dans d'autres conditions exp6rimentales. Soit D 1 la valeur de la dose de rayons X qui donne une survie de 37% pour les bact6ries ou pour I'ADN transformant irradi~s s 0 ~ en saline; soit D' sa valeur clans les m~mes autres conditions. Nous avons construit, pour chaque milieu, les diagrammes des rapports G'/G 1 et JD'/D 1 en fonction de la temperature d'irradiation.La comparaison de ces diagrammes montre que: