The antiestrogen tamoxifen is thought to antagonize the effects of estrogens by competing with them for estrogen receptor (ER) binding. However, tarnoxifen can also reverse multidrug resistance, synergize with cisplatin cytotoxicity, and inhibit growth in ER-negative lung cancer cells. In addition to ERs, rat and human target tissues contain a second binding macromolecule termed the type II estrogen binding site (type II EBS). It has been shown that tamoxifen and flavonoids, a widely distributed class of natural substances with a variety of biologic actions, bind to type II EBS and inhibit the growth of several tumor cell types. At present, conflicting data about ERs and an absence of data about type II EBSs exist for lung tumors. We have tested non-small-cell lung carcinoma cell lines and primary tumor cells for the presence of ERs and type II EBSs and have evaluated the effects of tamoxifen and quercetin (pentahydroxyflavone) on the growth of these cells. Using a whole-cell assay and nuclear and cytosolic radiobinding experiments with [3H]estradiol as tracer, we have found that SK-LU1, SW900, ChaGo-K-1, H441, H661, and A549 cells, as well as primary tumors, bind estrogen specifically. This binding results mainly from the presence of a large number of type II EBSs, whereas ERs are absent or present at low concentrations. Type II EBSs bound tamoxifen and quercetin with similar affinity. Cell counts and a thymidine incorporation assay showed that both compounds inhibit cell growth in a concentration-dependent manner at concentrations ranging from 10 nM to 1 microM. Neither ipriflavone, an isoflavone, nor rutin, the 3-rhamnosylglucoside of quercetin, bound type II EBSs or inhibited cell growth. These findings suggest that tamoxifen and quercetin could regulate lung cancer cell growth through a binding interaction with type II EBSs. This mechanism could also be active in vivo, in that we have observed that nuclear and cytosolic type II EBSs were present in all primary lung cancers tested (n = 12), and that tamoxifen and quercetin were effective in inhibiting in vitro bromodeoxyuridine (BrdU) incorporation and proliferation-cell nuclear antigen expression by neoplastic cells in these cancers.