Chalcone or 1,3-diphenyl-2-propen-1-one which is widely distributed in higher plants and is considered to be a precursor of all flavonoids, was studies in terms of its anti-tumor growth activity in vitro and in vivo. Mice given the chalcone, which was suspended in the drinking water, showed a statistically significant rise in percent survival: 66% at the 35th day for the chalcone-treated animals vs. 30% for the control ones. The chalcone also gave significant suppression of growth of HCT-15 and Meth/A cells in vitro. The dose of 50% reduction in growth was between 2.5 micrograms/ml and 0.6 microgram/ml for the HCT-15 cells and less than 10 micrograms/ml for the Meth/A cells. Histograms obtained by flow cytometric examination showed an elevated region between the diploid and the tetra-ploid nucleoidal peaks. A third peak heavier than the tetra-ploid peak was also observed, which peak was not observed in the control. The percentage of cells in the S phase was significantly raised. Microscopically, large nucleated cells were observed following addition of the chalcone. These results suggest that the chalcone induced abnormal DNA synthesis and mitosis in the cultured cells.