THE first attempts to utilize radium in therapeusis and experimentation were made without any importance being attached to filtration. The question was to determine the properties of a new substance. Those who were so fortunate as to possess a parcel of this substance used it in the form in which it had been supplied to them. The filtration was generally limited to the glass wall of the container in which the precious radium was sealed, It was with such a glass tube that, at the instigation of Pierre Curie and with radium furnished by him, Danlos (4) made the first attempts at treatment. In order to use more efficiently the very small quantities of radio-active substances then available, Strebel (26) originated the idea of placing the radio-active units in the very substance of the tumors to be destroyed, the introduction being carried out by means of a trocar. Among other advantages, the author attributes to this technic those of respecting the integrity of the skin and mucous membranes, of allowing prolonged irradiation, and of utilizing to better advantage the soft, and especially the beta rays. Another ingenious procedure which enabled one to use a larger proportion of rays and even some of the alpha rays, was conceived by Pierre Curie and first employed by Danlos (3). This consisted in spreading the radium salt on a small, flat, metallic surface and fixing it there with a varnish, The object then sought was to make available as much as possible of the energy furnished by the radio-active substance. Adoption of Filtration for Units of Radium Salts To the preceding conception, which was generally admitted for several years, came to be opposed that of Dominici, which was the fruit of physical and biologic experimental investigations. As early as 1907 this author (9) attributed to the very absorbable rays the necroses so frequently observed at the site of application and to the penetrating gamma rays the principal curative action; he proposed to separate these two groups of radiation by filtration. He wrote: “The results of this technic demonstrate that gamma radiation of low intensity, acting for a long time, can bring about the rapid regression of malignant tumors by avoiding all tissue changes other than those occurring in the tumor.” His experiments, carried out in collaboration with Barcat, confirmed his conception (10.) The application for a few minutes, to the surface of the skin of guinea pigs, of a radium plaque containing 25 milligrams of radium sulphate and emitting a composite radiation, provoked a superficial ulceration of the epidermis, followed by the permanent loss of hair and pigment. If the irradiation is prolonged a few hours the necrosis extends to the true skin; such treatment perforates the ear of a rabbit like a punch.