Agaricus bisporus is one of the most commonly cultivated mushrooms. During the course of the studies on nitrogenous constituents of the mushroom, several unidentified ninhydrin-positive compounds were found in the ethanol extract of the fruit bodies. Two of the unidentified compounds (M-I and M-II) were characterized in this paper. M-I was isolated via column chromatography, crystallized, and identified as L-saccharopine through elemental analysis, chromatographic behaviors, IR-spectra, and strong-acid treatment. M-II was purified by preparative paper chromatography, hydrolyzed to equimolecular amounts of glutamic acid and glycine, and identified as ƒÁ-L-glutamylglycine via N-terminal analysis, chromatographic behaviors, and configuration determination of the glutamic acid moiety. By the present experiments, the occurrence of L-saccharopine and ƒÁ-L-glutamylglycine was demonstrated. Though these compounds were identified in several organisms, this is the first report on their isolation from A. bisporus. The concentrations of both compounds are presented in Table II, and the mushroom seems to be a good source of L-saccharopine.