Purpose: To evaluate the radioprotective activity of C3, a regioisomer of water-soluble carboxyfullerene and a potent free radical scavenger, on both normal and tumour cells. Materials and methods: The murine committed bone-marrow stem cells for both granulocytes and monocytes (GM-CFC) were used to represent normal cells. For tumour cells, routine Ehrlich ascites tumour cells grown in regular tissue culture (EAT-T) and in the peritoneal cavity of CD1 mice (EAT-PC) and human HeLa cells were used. Cells were preexposed to varying concentrations (1-100μg/ml) of C3 at 37°C for 30 min before they were irradiated. Clonogenic assays were used to determine survival. The protection factor (PF), defined as the ratio of survival with and without C3, was then determined. Results: C3 protected GM-CFC in a concentration-dependent manner up to 50/μg/ml, and no additional protection was seen at 100μg/ml. The PF was 1.77 when bone-marrow cells were pre-exposed to 50/μg/ml of Cs before they were irradiated with 2 Gy. The value of PF increased to 2.38 when 4 Gy was used. In sharp contrast, C3 exerted less radioprotective effect on tumour cells. The PF values were 1.07, 1.43 and 1.07 for EAT-T, EAT-PC, and HeLa cells, respectively, when 2 Gy was given in the presence of 50μg/ml of C3. These values increased to 1.40, 1.75 and 1.27, respectively, when 4Gy was given. The dosemodifying factors at 10% survival were 1.37 and 1.15 for GM-CFC and EAT-PC, respectively. Conclusion: C3 exhibits a radioprotective effect on a class of normal haemopoietic progenitor cells. It also protects tumour cells, but to a lesser degree. It appears that C3 and other watersoluble fullerenes have a potential to be a new class of cytoprotectors.