Since our interest extended to the elucidation of biological functions of mushroom pigments (an unexplored area), we undertook the first investigation on the isolation and characterization of a pink pigment in the edible mushroom Pleurotus Salmoneostramineus L. Vass. The pigment was isolated from the mushroom chromoprotein (purified by repeated gel filtration of the water extract of the mushroom, followed by acetone precipitation of the glycoprotein containing metals and evaporation of acetone from the filtrate). Its structure was determined as 3H-indol-3-one (indolone) with the molecular formula C8H7NO by spectroscopic analyses (UV-vis, IR, MS, NMR) and confirmed by chemical synthesis. Indolone decomposes slowly in solution but is stable in crystalline form and in the chromoprotein; it is also stabilized by forming an inclusion complex with α-cyclodextrin, and the reconstituted chromoprotein (from indolone and the white glycoprotein containing metals) shows a bathochromic shift. We observed oxygen generation when an aqueous solution of the chromoprotein and the glycoprotein with β-structure was intermittently irradiated with a tungsten lamp at ambient temperature; no oxygen was generated without indolone. Thus, indolone plays a very important role in the photochemical generation of oxygen from water, suggesting its possible involvement in photosynthesis. Intensive investigation is underway to elucidate the structure of the proteins and reveal the dark reactions in this system.