A new pigment causing intense photosensitivity in rats was isolated from alcohol-treated Chlorella cells and identified as 10-hydroxypheophorbide a by chemical analysis, chromatography, and visible, infrared, nuclear magnetic resonance and mass spectroscopy methods. Whenrats administered orally with this pigment were exposed immediately to the visible light, signs of the intense photosensitivity including death occurred after a few hours ofphotoirradiation. The photosensitizing activity of this pigment in rats was markedly higher than those of pheophorbide a from Chlorella cells and pyropheophorbide a from pickled greens. Chlorophylls a and b, pheophytm a, and methyl and ethyl pheophorbides a were inactive under the same experimental conditions Microalgae containing chlorophylls (Chl's), such as Chlorella, Scenedesmus and Spirulina, have been studied for manyyears for utiliza

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