The application of energetic heavy particles, such as neutrons, protons, and alpha particles for treatment of neoplastic and neurologic diseases is of recent development. These particles, as compared to electrons, are in fact very heavy, and they possess unique advantages over x-rays in depth of penetration and lack of scattering in tissues. As an example, 340-Mev protons and 910-Mev alpha particles from the Berkeley 184-inch cyclotron can be made to travel in a pencil-like beam which gives an almost uniform distribution of radiation along its path and can be made to result in destruction in a predetermined, small, deep-lying structure. Furthermore, these mono-energetic heavy particles have a uniform and well defined range of penetration, producing maximum ionization just before stopping in tissues, producing the so-called Bragg effect. The range of the 910-Mev alpha particles in tissue is about 22 gm./cm.2 The present report will describe a preliminary experimental investigation of the RBE of 910-Mev alpha particles from the Berkeley cyclotron, based on mouse ascites tumors. For this purpose, TA3 transplantable ascites tumor,3 which grows progressively in and finally kills A/HeJ and A/J inbred mice, was chosen. The tumor from a freshly killed animal was diluted with normal saline to a known concentration of cells, and the resulting suspension was placed in a test tube for irradiation. For Bragg peak studies, special plastic chambers 1 mm. in depth were used. After irradiation, the tumor suspension was recovered from the test tubes and the plastic chambers, counted, and transplanted into a group of 5 to 10 A/HeJ mice. Each intraperitoneal inoculum consisted of 500,000 irradiated tumor cells and 50 per cent tumor takes was used as the end point. The mice were fed with Simonson Laboratory white diet and water ad libitum and were observed for six weeks after tumor transplantation. Radiation and Dosimetry The roentgen radiation was from a Philips unit, with the following technical factors: 200 kv, 15 ma, added filtration 0.25 mm. Cu and 1 mm. Al, h.v.l. 1 mm. Cu. For the alpha particle irradiation the 910-Mev beam was deflected from the cyclotron, magnetically sorted, and collimated by the quadruple focusing method. Following its emergence from the vacuum collimating system into air, it consisted of nearly parallel streams of particles in a homogeneous beam nearly 2 inches in diameter. The dose rates ranged from 50 to 100 rads per minute for roentgen irradiation and 500 to 1,000 rads per minute for alpha irradiation. For oxygenated and anoxic experiments pure oxygen and nitrogen, respectively, were bubbled through the tumor suspension for approximately fifteen minutes before and continuously during the irradiation. Results The experimental data have been briefly summarized in Figure 1.