Photofrin-mediated PDT was applied to malignant (A549 and MCF-7) and normal (HUV-EC-C) cells. The cells were incubated for different lengths of time after PDT. The cell responses to the therapy were examined by changes in SOD activity, phototoxicity, and mode of the cell death. PDT induced dynamic changes in SOD activity. Initially, an increase in SOD activity was observed, and after 6 hours of culture it decreased to the control level. Results obtained from MTT and the comet assay indicate that PDT caused immediate cell death via apoptosis in the A549, MCF-7, and HUV-EC-C cell lines. Our studies confirm that SOD is involved in the response of both cancer and normal cells to PDT.