Among the physical factors demontrated to affect the biological effect of X-irradiation are the qualitative differences between X-rays and the direction of the exposure, i.e., dorsal and ventral vs. two-directional. The dose rate was known to affect results when repair of radiation damage occurred during prolonged exposure. Dose rate was also responsible for the difference in acute double tube, acute dorsal exposure, and consecutive exposures from both dorsal and ventral sides which were administered within short exposures of as little as 12 minutes. The observed difference between dorsal and ventral exposures was attributed to a dose reduction to vital hematopoietic tissue by absorption of X-rays as they pass through mouse tissue. Physical systems do not measure dose rate effects but may be used to demonstrate absorption of ionizing irradiation. In comparing the high energy irradiation from Cesium-137 with that of X-rays it was found that a higher dose of gamma radiation was required to produce the same biological damage to hematopoietic tissue, however, there appeared to be less gut damage with increased exposure to Cs-137 than with the lower dose of X-irradiation.