Recent developments in radiobiology have created a need for a more detailed description of physical exposure conditions. In particular, the relative biological effectiveness (RBE) is believed to be related to the linear energy transfer (LET) (1) of the charged particles traversing irradiated tissues. In fact, permissible exposure to ionizing radiations has been based on both absorbed dose and ion density or LET (2). In the case of uncharged primary radiations there is in general a complex relation between the externally incident radiation field and the LET distribution of the dose imparted by internally produced ionizing secondaries. However, Boag (3) has computed the LET distribution of protons liberated in water by neutrons. This work is of considerable intrinsic interest, since it discloses characteristic LET distributions which should be considered in the planning of radio-biological experiments. In addition, the data provided may be applied directly in practical cases whenever known neutron spectra are employed. On the other hand, direct measurement of LET distributions substantially obviates the need for any knowledge of the characteristics of the primary radiation.