The flavonoid quercetin inhibits the heat-induced synthesis of heat shock proteins (hsps) in a variety of cell lines. To determine whether quercetin could inhibit hsp expression in breast cancer cells, we used the human breast cancer cell line, MDA-MB-231. Treatment of these cells with quercetin decreased the heat-induced synthesis of hsp27 and hsp70. However, inhibition of hsp expression did not correspond with the reduced ability of heat shock transcription factors (HSFs) to bind DNA. Furthermore, while quercetin treatment inhibited HSF2 expression, it only slightly affected HSF1 expression in breast cancer cells. In contrast, quercetin inhibited both HSF DNA-binding activity and HSF expression in HeLa cells. Our studies suggest that quercetin's action is cell-type specific, and in breast cancer cells may involve regulation of HSF transcriptional activity, rather than regulation of its DNA-binding activity.