Cells exposed to several forms of stress, such as heat shock, transiently synthesize a group of proteins called heat shock proteins (hsps). Although manystressors other than heat shock are knownto induce hsps, inhibitors of hsp expression have never been reported. Here weshowthat quercetiri and several other flavonoids inhibit the synthesis of hsps induced by heat shock in two humancell lines, Hela cells and COLO320 DMcells. Quercetin inhibited the induction of hsp70 at the level of mRNAaccumulation. This is the first report to describe the inhibition of hsp expression by reagents.