The aim of this study was to investigate the possible relationship between the bystander effect and the low-dose hypersensitivity/increased radio-resistance phenomenon in BJ fibroblast cells taking as response criteria different end points of radiation damage such as cell survival, chromosomal damage (as detected by using micronucleus assay) and double strand breaks (DSBs) of the DNA. Although gamma-H2AX foci were observed in confluent bystander BJ cells, our data suggest that X-irradiation does not lead to a significant rate of DSBs in bystander cells. Thus, neither bystander effect induced unstable chromosomal aberrations nor bystander effect induced DSBs are sufficiently pronounced to explain the apparent relationship between bystander effect and low-dose hypersensitivity. The experiments described here suggest that the hyper-radiosensitivity phenomenon might be related to bystander factor induced cell inactivation in the low-dose region (lower than 1 Gy).