During the past fifteen years a number of experiments have been done to compare the biological effectiveness of neutron and electromagnetic radiations; however, no consistent pattern has been evident in the various organisms and different radiations used. The reported ratios of relative biological effectiveness (RBE) of neutron and -y-radiation have varied from less than 1 to well over 20, depending on the type of biological specimen used and the response observed. Even within a single species (the mouse) and with mortality as the end point, reported ratios differ by a factor of 5 or more. A discussion of possible reasons for these apparent discrepancies can be found in the critical reviews of Aebersold (1) and Boag (2). It has been well established, however, that identical neutron radiations from the same source differed in their effect on similar tissues in the same species as well as for different responses in different species (3). The experiments reported here were designed to evaluate experimentally the RBE of fast neutrons by comparing their effectiveness with that of 250-kvp X-rays in CF1 mice. A single, total-body exposure was employed, and end points observed included 30-day mortality, as well as weight changes in the spleen, thymus and small intestine. All biological experiments were carried out in close collaboration with a program of physical measurements, and physical characteristics of the beam and dosimetric considerations are reported in detail in a separate communication (4). Other studies carried on concurrently, using the same neutron source, include the effects of fast neutrons on the dog (5) and preprotection studies in mice (6).