Survivin, a member of the inhibitor-of-apoptosis family is an essential protein for regular mitosis and is involved in an anti-apoptotic pathway. In some studies, an association between survivin expression and radiosensitivity has been described for tumor cells, but the relationship between p53 and survivin regarding radioresistance remains to be clarified. In order to increase the effect of irradiation on two sarcoma cell lines, A-204 with wt-p53 and US 8-93 with mt-p53, siRNA was applied to knock down survivin expression. The effects of combined treatment of siRNA treatment and irradiation were investigated by clonogenic survival assay, measurement of activity of caspases 3 and 7, Western blot hybridization for survivin and p53, and morphological analysis of apoptosis. Survivin knock down caused radiosensitization in the cell line A-204 (wt-p53) with an enhancement factor of 1.8 at 2 Gy (p=0.05) and 2.5 at 4 Gy (p=0.02), respectively. No radiosensitization was found in the cell line US 8-93 (mt-p53), when clonogenic survival was analyzed. These findings were supported by an increase in activity (up to 5.2-fold) of caspases 3 and 7 in cell line A-204 (wt-p53), but not in cell line US 8-93 (mt-p53) after a combined treatment of siRNA and irradiation. Our findings suggest that the wt-p53-caspase pathway is of importance for the radiosensitization induced by targeting survivin, which may have an impact on future gene therapeutical treatments.