The work is to study the bystander effects and associated mechanisms by irradiated conditioned medium (ICM) method. Splenic natural killer (NK) cells were obtained from healthy male ICR strain mice. ICM irradiated to different doses with ,60,Coγ-rays was used for culturing Yac-I Lymphoma cells. The degree of injury of latter by activated NK cells was observed. And effect of ICM on splenic lymphocyte proliferation was detected. Some samples of the culture media were pretreated 1h before the irradiation with 1% DMSO, which is a scavenger of reactive oxygen species (ROS), so as to investigate possible mechanisms of a radiation-induced bystander response. The results showed that severer injury was induced in Yac-I cells cultured in ICM than the control (without irradiation), as shown by increased sensitivity to murine splenic NK cells (,p,<,0.05). And the splenic lymphocyte proliferation decreased significantly than the control (,p,<,0.05). Culturing Yac-I cells in DMSO-pretreated medium considerably reduced the activation of NK cells. The result indicates the existence of bystander effects. It can be expected that DMSO can partly suppress ROS-induced bystander effect. This suggested ICM of Yac-I cells can trigger bystander effect. ROS may play an important role in radiation-induced bystander effect that can be partly suppressed by pretreatment with DMSO. ROS-induced bystander effect is seen as an adjunct mechanism but not as an essential one.