Melanoidins, a group of plant-produced melanins, were extracted from miso and shoyu (soy sauce), which are products of soybeans and widely consumed as food in oriental countries. The molecular weight of these melanoidins was approximately 5600 as revealed by Sepharose CL-4B column chromatography. The melanoidins were cultured together with HCT-15 cells derived from human colon carcinoma and AGS cells derived from human gastric carcinoma. Significant cell growth suppression was observed by incubating the cells with these melanoidins. The 50% growth suppression dose ranged between 100 micrograms/ml and 25 micrograms/ml, that is, between 2 x 10(-5) mM/ml and 0.5 x 10(-5) mM/ml. Flow cytometric analysis suggested that these melanoidins blocked the S phase and G2 to M phase of the cell cycle.