Chemical modifiers (radiosensitizers) are used in order to increase the efficacy of radiotherapy. The use of Photodynamic Therapy for tumor treatment, especially with Photofrin II, is also known. At present, no chemical modifier has been found to act as a selective radiosensitizer. Experiments using several series of cell lines were performed; human bladder cancer cell line (RT4), colon adenocarcinoma cells (HT-29), and the glioblastoma cells (U-373 MG) were investigated, with and without incubation with Photofrin II, before irradiation. The irradiation was performed using doses ranging from 0 to 8Gy. Colony forming tests were applied to determine the efficiency of Photofrin II as a radiation sensitizer in comparison to irradiation alone. Two of the cell lines tested, cultures of the RT4 and U-373 MG, treated with Photofrin II prior to radiation, showed cell survival lower than cultures untreated with Photofrin II but irradiated under identical conditions. For the HT-29 cells, the results did not differ between the two groups (with and without Photofrin). The results of this study showed that Photofrin II can act, under certain conditions as a tumor radiosensitizer. (C) 2003 Published by Elsevier Inc.