The rice koji for sake brewing usually shows white color, but it sometimes turnsto brown during storage in the air. The precursor of browning substances in rice koji was studied by comparing two kinds of koji made by culturing two kinds of A. oryzae on steamed rice; one is strain RIB 128: which always makes koji brown, and another is strain RIB 647 which always does not brown.<BR>When the koji was steeped in water at room temperature and filtered, both the filtrate and the residue gradually colored brown in the presence of air and finally became black.Browning degrees of the filtrate were compared between the kind of strain and / or the temperature of the steeping water as shown in Fig. 2. The coloration in both filtrate and residue was increased atalkaline pH of steeping water as shown in Fig. 3. High correlation was obtained statistically between the color of filterate and the browning degree of the residual matters as seen in Table 1 Therefore, itis clear that the precursor of brow ing substances of rice koji is soluble in water. Studies were hereafter carried out by using the filtrate of rice koji made by culture of RIB 128, and the results described below were obtained.<BR>1. The precursor of browning substances in the filterate was adsorbed onto alumina at pH 8. 5 and eluted with 0.3 N acetic acids as shown in Figs. 1, 4, 5 and Table 2. The eluate obtained by acetic acid from alumina column was collected and evaporated under reduced pressure at about 45°, followed by thin-layer chromatography (TLC). Three spots on TLC became gradually in the air. On the conirary, no spots on TLC were found in the eluate obtained from the filterate of rice koji made by culture of strain RIB 647.(Fig. 6) From the densitometric analysis of three kinds of spot-substance, the substance of R_f O. 47 was regarded as a main precursor among three spot-substances as shown in Table 3.<BR>2. The eluate by acetic acid from alumina column was subjected to chromatographyby Dowex 50-X 8, followed by successive elution with water, 1 N HCl and 4 N HC1, and the spot-substances of R_f 0.96, 0.47 and 0.56 were sesarated as shown Fig. 7.<BR>3. Comparing the R_f of TLC and color reactions of the main spot-substance with those of DLdihydroxyphenylalanine (dopa), the main spot-substance was found to be similar to the latter substanceas shown in Table 4 and 5.<BR>4. The chromatographic pattern of the main spot-substance examined with an aminoacid analyzer was the same as that of DL-dopa, which was eluted between leucine and tyrosine as shown in Fig. 8.<BR>5. UV-absorption maximum of the main spot-substance was found to lay at 282 mμ, and it was the same as that of DL-dopa.(Fig. 9)