Among chlorophyll derivatives (CpD-A, -B, -C, and -D) extracted from silk worm (Bombyx mori) excretas, CpD-A was extensively studied to clarify its role as a "photosensitizer" for photodynamic therapy (PDT) of tumors in vitro. It was found that CpD-A was photoreactive both in itself and also in its cell bound forms. The cell bound CpD-A produced fluorescent light and singlet oxygen following the exposure to lights of varied wave length. Among them, lights of near 650 nm, which was the maximum absorbance band, efficiently activated CpD-A following the application of only 10 minutes of irradiation. CpD-A was found to have specificity for the human and mouse tumor cells regardless of their species difference. A higher intensity of fluorescence and a larger amount of CpD-A were found in the tumor cells as opposed to the intensity found in normal cells. Only 10 minutes of light irradiation of the CpD-A treated tumor cells resulted in their rapid and complete destruction within 2 hours of irradiation. Simultaneously, more than 80% of the normal human and mouse control cells remained alive after receiving treatment. These findings suggested that CpD-A produced by use of silkworm excreta could be used as a photosensitizer for PDT of tumors by the use of lights of near 650 nm.