The aim of this in-vitro study was to evaluate the combination of busulfan with radiotherapy on TE-671 human medulloblastoma cells since unexpected clinical toxicity of busulfan was reported during the treatment of brain tumors, suggesting a possible radiopotentiation. The cytotoxicity of busulfan was determined by using colony forming assays, and doses inducing growth inhibitions of 10, 20 and 50% were selected to be tested in association: 6, 12 and 32 mumol/l for busulfan and 0.5, 1 and 3 Gy for irradiation. All possible combinations were considered within this frame and the results showed that the combination of busulfan with radiotherapy exerted an additive effect without potentiation.