A quantitative comparison of the photodynamic effect of pulsed (copper vapor-pumped dye laser), continuous-wave laser (He-Ne laser), and non-coherent red light (filtered from a halogen lamp) to the liver in normal mice has been carried out. The mice (each group containing 18 - 20 mice) received hematoporphyrin derivative in a dose of 10 mg/kg iv 24 hours prior to light irradiation. The mouse livers were directly irradiated with different red light in a dose of 5, 10, 25, 50, or 100 J/cm2, respectively. Forty-eight hours later the mice were killed, and the depth of liver necrosis was measured using a computer image-analysis system. No necrosis was found in the control liver irradiated with 500 J/cm2 alone. The depth of photodynamic necrosis showed a light dose-dependent response. The mean depth of necrosis of all groups were compared statistically. The Cu-dye laser showed the best effect while the non-coherent light showed the poorest. There were significant differences between non-coherent light and laser irradiated groups, but not between Cu-dye and He-Ne laser groups. The results indicate that among the examined light sources, the Cu-dye laser is most suitable to photodynamic therapy (PDT) of tumors. However, the halogen lamp with a special filter device may still be occasionally used as a light source in PDT if needed.