AINSWORTH, E. J., JORDAN, D. L., MILLER, M., COOKE, E. M., AND HULESCH, J. S. Dose Rate Studies with Fission Spectrum Neutrons. Radiat. Res. 67, 30-45 (1976). Fission spectrum neutrons produced by the JANUS reactor were used to test the hypothesis that no effect of dose rate or exposure time occurs when mouse lethality or hematopoietic injury is evaluated. Reduction of the dose rate from 13.1 to 1.2 rad/min resulted in a small increase in LD50/30 and LD50/1. Whereas no dose rate effect was found for cell killing (Do) of hematopoietic stem cells in the femur or spleen, mice that received a sublethal dose at 1.2 rad/min showed a small but reproducible advantage in repopulation of stem cells in the femur, and in reappearance of platelets in peripheral circulation. During a course of fractionated neutron irradiation, the femur stem cell content was higher in groups which were irradiated at 0.13 rad/min in comparison with others irradiated at 2.13 rad/min. Thus, the effects of fission spectrum neutrons are not totally independent of dose rate or exposure time based on the end points evaluated in these experiments. The effect of neutron dose rate on cell