Abstract
Using the primary beam of 250Kv X rays at 54-61 r/minute and minimal backscatter the LD50(3O) f[degree]r 110 daY old female CBA mice was 763 [plus or minus] 8 r with the beam entering dorsoventrally and 860 [plus or minus] 8 r with the beam entering ventrodorsally. With maximal backscatter values were 780 [plus or minus] 13 r and 850 [plus or minus] 9 r respectively. Differences in beam direction and degree of scatter account for only part of the difference in LD50(3O) reported from different laboratories.