We want to discuss:(1) the strange selective action of X-rays on epithelial tumors;(2) tumors produced by X-rays. All physicians have observed with the same interest mixed with surprise that X-radiation can kill the cells of a neoplasm without impairing the neighboring healthy tissues or even the tissues invaded by the tumor. But experiments on animals have shown that X-rays have a selective action among healthy tissues. For example, in our experiments on the testicle of rat, we have been able to destroy the germinal cells whereas the interstitial tissue and the sertoli syncytium were unimpaired. As a result of these experiments, it has been possible to formulate the following law: X-rays are more effective on cells which have a greater reproductive activity; the effectiveness is greater on those cells which have a longer dividing future ahead, on those cells the morphology and the function of which are least fixed. From this law it is easy to understand that roentgen radiation destroys tumors without destroying healthy tissues. It was quite puzzling when many and irrefutable observations demonstrated that these same roentgen rays capable of destroying malignant tumors in patients were also capable, at times, of producing in the tissues of the radiotherapist (who was previously apparently perfectly healthy), negligently exposed in the field of radiation, the appearance of tumors identical to the ones they can destroy. There is no longer any doubt about this point; X-rays have produced in radiolo-1 This paper has been translated by Dr. Gilbert H. Fletcher of the MD Anderson Hospital and Tumor Institute, Houston, Texas, at the request of several members of the Radiation Research Society. It was originally published in 1906 in Comptes rendus des seances de l'academie des sciences, Vol. 143, pp. 983-985. It is published here because of the historical value and the general interest in the principle as outlined by Bergonid and Tribondeau. Dr. Fletcher has pointed out that" this year is a fitting time to remind readers of Radiation Research of this early contribution, as this is the fortieth anniversary (1919) of the Curie Foundation and Radium Institute. Much of the fundamental radiobiological and clinical work of Regaud, Lacassagne, Coutard, and Ferroux is a development of Bergonie and Tribondeau's intuition."