Radiation induced by-stander effect (RIBE) is a poorly understood phenomenon wherein non-irradiated cells show evidence of DNA damage, genomic instability, mutation and apoptosis bothin vitro and in vivo. We have observed that chromatin fragments (Cfs) that are released from dead cells resulting from radiation treatment are one of the key factors responsible for RIBE. We employed the widely used trans-well plate system wherein NIH3T3 mouse fibroblast cells grown on trans-well filters were irradiated using a Co60 g-ray source, and RIBE in the form of dsDNA breaks represented by H2AX activation were analyzed in recipient NIH3T3 cells grown on tissue culture plates. gH2AX signals increased approximately 3-fold following 6hr incubation of irradiated cells with the recipient cells confirming activation of RIBE. However, the number of gH2AX signals dropped dramatically when donor cells were treated at the time of irradiation with anti-histone antibody complexed nanoparticles (CNPs), which are known to specifically bind to and inactivate Cfs. Since blood levels of Cfs increase following radiotherapy, Cfs may induce RIBE in cells of various organs of the body and be responsible for side-effects of radiotherapy for cancer.