Fast neutrons are at present being considered as a possible alternative to more conventional radiotherapy in the treatment of cancer. The relative biological effectiveness (R.B.E.) of this high linear energy transfer (L.E.T.) radiation is known to depend on the size of the dose (1,2). It is also of considerable importance to the therapist to know whether the R.B.E changes markedly with the particular tissue being irradiated. Basic Differences Between Survival Curves Produced by X Rays and Fast Neutrons: There are three basic differences between the survival curves produced by exposing mammalian cells to low- and high-L.E.T. radiations, e.g., x rays and fast neutrons: (a) At all dose levels, the R.B.E.3 for fast neutrons relative to x rays remains greater than unity (Fig. 1). (b) The shoulder of the survival curve for cells irradiated with x rays is normally larger than that for irradiation with fast neutrons, i.e., the extrapolation number is larger with x rays than with neutrons. A consequence of this di...