Xanthohumol, a prenylated chalcone of the Hop plant (Humulus lupulus L.), has been reported to suppress tumor growth. 4-hydroxychalcone and isobavachalcone are chalcone derivatives and they have similar structure with xanthohumol. In the present study, we investigated the cytotoxic activities of chalcone and its derivatives, 4-hydroxychalcone, xanthohumol, and isobavachalcone, in MCF-7 and adriamycin resistant MCF-7 (MCF-7/ADR) breast cancer cells and HT-1080 fibrosarcoma cells. In a cell viability assay using 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide (MTT) reagent, chalcone and 4-hydroxychalcone decreased cell viability in HT-1080 cells, but not in MCF-7 and MCF-7/ADR cells. Isobavachalcone showed similar cytotoxicity in HT-1080 cells, and only limited cytotoxicity in MCF-7 and MCF-7/ADR cells at very high concentration (50 ${mu}M$). In contrast, xanthohumol showed concentration-dependent cytotoxicity in MCF-7, MCF-7/ADR, and HT-1080 cancer cells. Taken together, the structure-activity relationship of chalcone and its derivatives indicate that chalcones may be valuable cytotoxic compounds against selective cancer types.