The investigation of physical factors affecting the success of bone marrow transplantation experiments was continued and include both radiobiological effects of the direction of the exposures and subtle changes in physical factors that alter the survival of irradiated inbred mice with and without marrow graft. Basic concepts of radiation biology were demonstrated to be invalid and physical factors, considered of little consequence when applied to biological systems, were critical for the reproducibility of these experiments. For example, exposure-rate effects attributable to cell proliferation and repair during protracted and fractionated exposures also occurred over short exposure times as a result of repair of sublethal radiation damage, i.e., identical results were obtained when simultaneous 2 directional exposures were administered over the same time span as the dorsal exposure. In addition, sequential alternate exposures were less effective than either two direction or unidirectional exposures when treating the circulating and fixed cells of the hematopoietic system. X-rays are of different wave lengths and the "quality" of the X-rays have been defined in terms of the filtration by copper and aluminum and the half value layer. These crude methods of "definiting" the quality of X-rays have been demonstrated to be inadequate. New more precise methods of determining these parameters must be found if experimental conditions are to be met and experimental data reproduced.