Ataxia telangiectasia mutated protein , and ataxia telangiectasia and Rad3 related DNA-dependent protein kinase have also been implicated. Recently, some laboratories have suggested that the presence of serotonin in the serum is one of the key factors involved in the bystander effect, however this finding has been disputed. Most bystander effect studies have been performed using x-rays, gamma rays and alpha particles, however, little has been done concerning the effects of neutron radiation. Such information might be important for risk estimation in response to neutron exposure. No conclusive cytogenetic evidence exists to support or refute the existence of non-targeted effects in cellular responses to neutrons. A bystander effect following neutron exposure has been observed in Chinese hamster ovary cells, but no effect was seen in zebrafish irradiated in vivo. There are no available cytogenetic data concerning the bystander effect in human cells in response to neutrons. Here we used the cytokinesis-block micronucleus assay to address the question whether neutrons induce a bystander effect in normal human lymphoblastoid cell lines. The experiments described here provide no cytogenetic evidence that fast neutrons are capable of inducing a bystander effect through medium-borne factors. The neutron beam used in these experiments was contaminated with photons, necessitating parallel evaluations to determine whether there is a positive or an inhibitory effect of these photons on the results of the neutron experiments. Our observation of an absence of a bystander effect following doses of photons that are associated with exposure to neutrons confirmed that there is a lack of a bystander effect in response to neutrons, regardless of the presence of photons. Wang et al. have suggested that neutrons might suppress gamma ray-induced bystander signaling. They measured apoptosis and cell survival in zebrafish that received bystander signals from fish that were directly irradiated with neutrons. Since the doses of photons that contaminated these neutron exposures exceeded the minimum threshold for inducing a bystander effect, Wang et al. suggested that the gamma ray-induced bystander effect might have been suppressed by the neutron exposures. With our data it is difficult to determine whether neutrons have the ability to suppress any bystander effect produced by photons because a source of uncontaminated neutrons is not available. The results shown here suggest that contaminating photons are not a likely confounding factor that interfered with the ability to detect a neutron-induced bystander effect. Different responses to neutrons were observed in the two cell lines we used. GM15510 cells cultured in ICCM from neutron irradiated cells showed substantial variation in micronuclei frequencies but no consistent dose-related response. In contrast, GM15036 cells had micronuclei frequencies lower than the corresponding dose control value.