Abstract: An antioxidant for use in treatment for a radiation-induced bystander effect to a transplanted cell in a host is provided. The antioxidant is N-acetyl-L-cysteine, sulforaphane or resveratrol, or a combination thereof. The RIBE on transplanted human hematopoietic cells impaired the long-term hematopoietic reconstitution of human HSCs as well as the colony-forming ability of HPCs, and the RIBE-affected human hematopoietic cells showed enhanced DNA damage responses, cell cycle arrest and p53-dependent apoptosis, mainly due to oxidative stress. Taken together, these findings suggest that RIBE impairs human HSCs by oxidative DNA damage. The present disclosure provides definitive evidence for RIBE in transplanted human HSCs and further justifies the necessity for conducting clinical trials to assess the ability of multiple antioxidants to improve the efficacy of HSC transplantation for patients with hematological or non-hematological disorders.