Photodynamic therapy (PDT) is a new form of cancer treatment that uses the localized delivery of light and a photosensitizing drug, which is selectively retained in tumor tissue, to cause photochemically induced cell death. Although PDT mediated by the sensitizer Photofrin (Ph-PDT) is currently in Phase III trials for a number of human cancers, the exact mechanism(s) involved in PDT induced cytotoxicity is not fully understood. Also, Photofrin has a number of drawbacks including extended cutaneous photosensitization and low absorption in the red region of the spectrum. This has lead to the search for improved sensitizers. In vitro, tumor cells resistant to PDT have been developed from PDT sensitive cell lines to examine the mechanism(s) of PDT action. In this work, the sensitivity of RIF-1 murine fibrosarcoma cells and RIF-1 derived Ph-PDT resistant RIF-8A cells was examined following several damaging agents including PDT mediated by the novel Ruthenium phthalocyanine photosensitizer JM2929 (JM2929-PDT), UV, gamma-radiation, and hyperthermia. Gamma-radiation sensitivity of two other RIF-1 derived Ph-PDT resistant variants, CPR-C1 and RIF-P16CL8, was also examined. RIF-8A cells showed cross resistance to UV but increased sensitivity to gamma-rays compared to RIF-1 cells. RIF-1 and RIF-8A cells showed similar sensitivity to JM2929-PDT and hyperthermia. It is possible that Ph-PDT induces a "UV -like" component of damage and/or there is some overlap in the pathways for the repair of UV and Ph-PDT induced damage, but not JM2929-PDT, hyperthermia, and ionizing radiation damage in RIF-1 and RIF-8A cells. A cross resistance to gamma-rays was observed for CPR-C1 but not RIF-P16CL8 cells. Since Ph-PDT resistant CPR-C1 cells, but not RIF-8A cells or RIF-P16CL8 cells, show a cross resistance to gamma radiation, these results suggest that the cellular changes required for RIF-8A, RIF-P16CL8, and CPR-C1 cells to become resistant to Ph-PDT are different. Survival of RIF-1 and RIF-8A cells following gamma-rays in the presence of either Photofrin or JM2929 was also examined. Results suggest sensitization of RIF-1 cells, but not RIF-8A cells, to gamma-radiation in the presence of Photofrin. Gamma-radiation in the presence of JM2929 had no sensitizing effects on the survival of RIF-1 and RIF-8A cells. DNA repair of a UV-damaged reporter gene was also examined in untreated as well as Ph-PDT, JM2929-PDT, UV, cisplatin, and hyperthermia pretreated RIF-1 and RIF-8A cells. Results suggest an increased repair of UV damaged DNA in untreated RIF-1 cells compared to untreated RIF-8A cells. Ph-PDT, JM2929-PDT, and UV pretreatments resulted in an increased reactivation of a UV damaged reporter gene in RIF-1 cells compared to RIF-8A cells. Enhanced reactivation of a UV damaged reporter gene was not observed in either RIF-1 or RIF-8A cells following cisplatin or hyperthermia pretreatment. Enhanced expression of an undamaged reporter gene was greater in RIF-8A cells compared to RIF-1 cells following Ph-PDT pretreatment, but similar to RIF-1 cells following pretreatment with all other agents. These results suggest that the relation between survival, DNA repair of an actively transcribed gene, and transcriptional enhancement of an actively transcribed gene, varies in RIF-1 and RIF-8A cells depending on the damaging agent used. However, decreased reactivation of a UV damaged reporter gene in RIF-8A cells may be related to Ph-PDT and UV resistance seen in RIF-8A cells. / Thesis / Master of Science (MSc)