Type-II estrogen-binding sites (type-II EBS) have been demonstrated in the human lymphoblastoid cell line IM-9 using a whole-cell assay with (6,7-3H) estradiol (3H-E2) as tracer. Competition analysis showed that the anti-estrogen tamoxifen and the flavonoids quercetin and rutin competed for (3H)-E2 binding to type-II EBS. Growth experiments demonstrated that diethylstilbestrol (DES) tamoxifen (TAM), quercetin and rutin exerted a reversible dose-dependent inhibition of cell proliferation in the range of concentrations between 10 nM and 10 microM. The relative binding affinity of quercetin, rutin, DES and TAM for type-II EBS correlated well with their potency as cell growth inhibitors. Moreover, hesperidin, a flavonoid which does not bind to type-II EBS, was ineffective in inhibiting cell growth. Cell-cycle analysis showed that the growth-inhibitory effect of DES, TAM or quercetin was due to a blocking effect in the G0-G1 phases. Our results suggest that high estrogen and anti-estrogen concentrations and flavonoids may regulate IM-9 cell growth through a common mechanism involving a binding interaction with type-II EBS.