While nucleoplasmic bridges are formed when centromeres of dicentric chromosomes are pulled in opposite directions during mitosis. RBE values are known to depend on factors such as linear energy transfer, tissue type, the extent of biological damage, and dose. Knowing the RBE is important for radiation oncologists to determine the dose prescription and the most effective radiotherapy treatment plan for cancer patients. Yang et al. reported RBEs of 2.35 and 2.42 for fast neutrons in immature rat hippocampal cells, as determined by two different cell viability assays. Dagrosa et al. used the cytokinesis-block micronucleus assay and a cell survival assay in a human colon carcinoma cell line and observed an RBE of 4.4 for neutrons in boron neutron capture therapy. RBEs for neutrons as low as 4 to as high as 63 have been reported after measuring life-shortening responses in mice, apoptosis and induction of dicentrics in human lymphocytes. These numbers clearly indicate that the RBEs for neutrons vary with the biological system, neutron energy and the end-point. Our RBE values are within the range of what others have reported. In conclusion, we found no evidence for a bystander effect following exposure to fast neutrons or to doses of cobalt-60 photons equivalent to 5% of the neutron dose. As expected, a bystander effect was seen with high doses of photons, as evaluated by micronuclei frequencies and nucleoplasmic bridges. These results will facilitate refined estimates of the risk-benefit ratio of neutron therapy and may be valuable to those who are concerned about the health effects of exposure of space travel. We have also shown that these fast neutrons have a relative biological effectiveness of 2.060.13 for micronuclei and 5.862.9 for bridges compared to cobalt-60. Understanding the biological effects of neutrons may also enable more refined evaluations of the standards for radiation protection and safety. Calculation of the NDI was important to ensure that the number of binucleated cells was sufficient for enumerating micronuclei. Cells were then evaluated simultaneously for micronuclei, nucleoplasmic bridges and buds according to our “relaxed” criteria. Briefly, only binucleated cells with non-overlapping nuclei were evaluated. Micronuclei were required to be no more than one-third the size of the nuclei, and to be round or oval with smooth edges and stained the same color as the nuclei. Bridges were required to span the entire distance between the two nuclei. Buds were counted only if the stalk was thinner than the widest part of the bud. Since buds did not exhibit a consistent response for either cell line in any of the experiments, we have not included these data in this paper. For each treatment condition, at least 1000 binucleated cells were scored by trained observers. For any experiment, either one observer evaluated all the treatment conditions or the scoring was balanced between two observers such that each evaluated approximately equal numbers of cells for each.