A light sensitive substance, Hematoporphyrin Derivatives (HpD) is known to have an affinity with tumor cells, which are degenerated to necrosis by photoradiation therapy (PRT). Fundamental studies were performed to elucidate the mechanism acting on tumor cells and to evaluate efficiency when applied to gynecological malignancies. The SKG-1 cell line established from uterine cervical cancer was used in this study. No effects on cell growth inhibition (CGI) can be expected by single irradiation. Single HpD has some effects on CGI depending on the HpD concentration and length of treatment time. In PRT, a proportional increase in effects is obtained by extending the irradiation time. More than 90% effectiveness is obtained by irradiation for 5 minutes or more. In a cytomorphological study of the time course changes in cultivated cells subjected to PRT, prominent cytoplasmic changes are observed even immediately after PRT. Advanced changes in nuclei are seen within a few hours after PRT. Electron microscopic findings are mitochondrial swelling and destruction, ER distension, destruction of plasmamembrane and decreased density of nuclei as chromatin disappears. The DNA pattern of the cells 24 hours after PRT showed a shift in the histogram peak to diploid and a remarkable decrease in the number of high ploidy cells above tetraploidy. This indicates early degeneration of DNA after PRT.