We examined bystander cell death produced in T98G cells by exposure to irradiated cell conditioned medium (ICCM) produced by high-energy 20 MeV electrons at a dose rate of 10 Gy min⁻¹ and doses up to 20 Gy. ICCM induced a bystander response in T98G glioma cells, reducing recipient cell survival by more than 25% below controls at 5 and 10 Gy. Higher doses increased survival to near control levels. ICCM was analyzed for the presence of transforming growth factor α (TGF-α) and transforming growth factor β1 (TGF-β1). Monoclonal antibodies for TGF-α (mAb TGF-α) and TGF-β1 (mAb TGF-β1) were added to the ICCM to neutralize any potential effect of the cytokines. The results indicate that TGF-α was not present in the ICCM and addition of mAb TGF-α to the ICCM had no effect on bystander cell survival. No active TGF-β1 was present in the ICCM; however, addition of mAb TGF-β1 completely abolished bystander death of reporter cells at all doses. These results indicate that bystander cell death can be induced in T98G glioma if a large enough radiation stress is applied and that TGF-β1 plays a downstream role in this response.