Ever since x-rays were found to be therapeutically useful, radiologists have been striving for higher and higher voltages, because the greater the voltage the deeper the penetration of the incident radiation. In 1949 it became possible to purchase a multimillion-volt unit at the University of California School of Medicine in San Francisco because of the interest of the U. S. Atomic Energy Commission. The choice of a synchrotron producing 70-Mv x-rays resulted from several factors. Units of various kinds producing x-rays up to 31 Mv were then being tested or installed in other institutions. If a new energy level were to be investigated, it must be above 31 Mv. The General Electric Company had a 70-Mev synchrotron operating in their laboratories in Schenectady. With a synchrotron, 70-Mv x-rays could be obtained from a unit of the same size and weight as a 50-Mev betatron. Another consideration was that one of the authors (Stone) who had treated patients with high linear energy transfer (LET) radiation (with fast neutrons) with poorer results than from 1-Mv x-rays, wished to try very low LET radiation. This article, as the title implies, follows a paper reporting the physical aspects of the synchrotron (1), in which Adams recorded many of the technical details of the synchrotron's design and operating characteristics, and the physics involved in employing it in x-ray therapy. The unit was installed in 1951. Clinical trials did not start until 1956 because considerable engineering feats, physical studies: dosage measurements, and biological tests had to be performed before it was possible to treat patients safely. Suffice it to saythat the clinicians were supplied with: (a) a machine which operated so well only two days of planned treatments were missed in eight years, (b) dosimetry measurements so carefully worked out in rads the accuracy was beyond that normally possible with machines in clinical use, a collimation scheme which made it easy to treat carefully chosen regions with sharp limitations, and compensating filters for each shape of field constructed so that the isodose levels at the depth of the tumors were very flat. Dosage Considerations Once the machine was in excellent operating condition, the next question to be settled was the relative biological effectiveness (RBE) of the 70-Mv x-rays. Some information was available in reports on megavoltage x-rays from betatrons and linear accelerators, but these data were not convincing because they were not based on physical measurements of absorbed dose in rads or on uniformly planned biological studies. Kohn (7) and his collaborators solved this problem by a series of inter-laboratory comparisons employing the LD50 of yeast which revealed the RBE to be between 0.86 and 0.88 for x-rays from a 1-Mvp Maxitron, a 22.5-Mev betatron, the 70-Mev synchrotron, and γ-rays from Co60.