Quercetin is a known specific inhibitor of hsp70 synthesis and thus might be a potent agent for enhancing the selective cytotoxicity of heat on tumour cells. A comparative analysis of the effects of quercetin and five structurally related flavonoids on hsp90alpha, hsp70A, hsp60 and hsp27 gene expression was carried out using human myeloid leukaemia cells (HL-60). The cells were preincubated with 50 muM quercetin, kaempferol, myricetin, taxifolin, isorhamnetin, methylquercetagetin or 0.1% DMSO (controls) for 24 h at 37 degreesC before heat shock treatment (43 degreesC for 30 min). Total RNA was isolated from heat-stressed and unstressed cells and analysed by RT-PCR. Hsp27 gene expression was inhibited by flavonoids more strongly than other hsp genes investigated in heat stressed as well as in unstressed cells. Among the hsp genes tested, only hsp60 was expressed above control level under the influence of taxifolin. Members of the hsp70 and hsp27 families are highly expressed in breast and lung cancer and leukaemias and they play a role in the acquired resistance to chemotherapy or radiation therapy combined with hyperthermia. Therefore, hsps present potential targets for cancer diagnosis and treatment. The present structure/activity study indicates that position, number and substitution of hydroxyl groups of the B ring and saturation of the C2-C3 bond are important factors affecting flavonoid activity on hsp gene expression. This study could help provide a basis for further design of specific inhibitors of hsp gene expression.