The mechanisms of medium-mediated bystander effects on cell survival and micronucleus (MN) induction were investigated by co-cultivating unirradiated HSG cells with cells irradiated by X-rays or 290 MeV/u carbon beams. It was found that the survival of the irradiated cells exponentially decreased along with the dose, and that the plating efficiency (PE) of the unirradiated recipient cells was obviously more enhanced than that of the control cells. Moreover, MN was induced in the unirradiated recipient cells and its yield had a maximum distribution corresponding to the donor dose, which was different from the linear-quadratic dose response of the yield of MN in the irradiated cells. The treatment of PTIO, a scavenger of nitric oxide (NO), decreased both PE and MN of the unirradiated recipient cells to control levels. Moreover, nitrite was detected in the co-culture medium, and its concentration was related to the donor dose. These results indicated that NO was involved in the above mentioned medium-mediated bystander effects. In addition, an equation was deduced to well fit the induction of MN of the unirradiated recipient cells.