564 BOND, CRONKITE, SONDHAUS, IMIRIE, ROBERTSON, AND BORG exposure, all those considered yield reasonably flat or uniform depth-dose patterns (17, 17). The relationship of the midline tissue dose to the entrance air dose, for any exposure geometry, will vary considerably with beam energy, target-to-skin distance, and animal thickness. The shape of the depth-dose curves (essentially Hat) for all geometries except unilateral exposure is remarkably insensitive to these factors for radiations and exposure conditions commonly used for large animal irradiations (200- to 2000-kvp X-rays, Co® y-rays). As the beam energy becomes low (practically at about 100 kvp), or with animals of very large diameter (as with burros), the midline tissue dose becomes vanishingly small compared to the entrance air or entrance tissue doses, and the depth-dose curve is far from flat. This type of “energy dependence” of biological effect has been investigated quantitatively (17-/9). It should be noted that. although fallout y-radiation has been termed ‘soft,’ only a very small percentage of the primary beam ts below 100 to 200 kev under most practical circumstances (1). Thus the fallout y-raciation must be considered quite penetrating in terms of biological effectiveness. Correlation of depth-dose patterns with biological effect. From the depth-dose considerations outlined above, wide variations in the dose required for a given biological effect, expressed as air dose, would be expected with different exposure conditions. A glance at Tables I and II, in which large animal mortality data from the literature are collected, shows this to be true. The LD, values for dogs and swine are given in the tables in terms of entrance air dose, as well as in terms of the entrance, midline, and exit tissue doses. A better correlation between dose and effect would be expected if tissue dose is used unless (1) marked differences in the shape of the depth-dose pattern exist, (2) a true energy dependence ofbiological effect is present, or (3) strain differences in the degree of biological effect exist. If the LD0 values are considered for bilateral X-irradiation, in which the depth-dose curves are flat, the several LDio values obtained are remarkably close in terms of the midline tissue doses for both dogs and swine. Since the determinations were made by several investigators under different conditions, this indicates a marked lack of sensitivity of the LD, value on X-ray beam energy (over the range employed), TSD, animal strain used, or small variations in the essentially flat depth-dose patterns employed. Appreciable differences in LD5, expressed as midline dose, oceur only when the depth-dose ad mo er pattern is altered markedly (as with unilateral exposure), or when Co®y-irradiation is used. Unilateral exposure yields higher LDso values in the laboratory, as might be expeeted in considering the relatively little-exposed tissues on the distal side (see studies on the effect of spleen and bone marrowshielding in references 27 and 28). Expressing the LD5o as midline tissue dose does not bring the values for unilateral exposures underdifferent laboratory conditions into agreement, nor does it allow