Toxaphene is a complex mixture of chlorinated bornanes, bornenes, and bornadienes and was a heavily used insecticide in the United States until its use was restricted in 1982. There are conflicting reports regarding the potential for toxaphene to induce estrogenic responses in human and nonhuman animals. Due to the public concern over environmental estrogens, the estrogenicity of toxaphene was examined in a human breast-cancer cell assay, the MCF-7 focus assay, which is based on in vitro postconfluent cell proliferation and tissue restructuring. In this assay, 0.1-1 n M 17 b estradiol (E) produces maximum postconfluent proliferation and formation of multicel2 lular nodules or foci. Toxaphene was also tested for its ability (1) to bind the estrogen receptor (ER) in a competitive binding assay using recombinant human ER a (rhER) and in a whole-cell competitive ER binding assay, and (2) to alter the catabolism of E 2 in MCF-7 cell cultures. Results from the MCF-7 focus assay showed: (1) Toxaphene alone was not estrogenic between the concentrations of 0.5 n M and 10 mu M, (2) toxaphene in binary combinations with chlordane, dieldrin, or endosulfan (a or b) was not estrogenic, and (3) toxaphene was weakly antiestrogenic (it reduced the number of foci induced by 0.1 n M and 0.01 n M E2). Results from the competitive binding assays showed that (1) toxaphene alone did not bind rhER or ER in MCF-7 cells, and (2) toxaphene in binary combinations with other pesticides did not bind rhER. Results from the growth assay and radiometric analysis of E2catabolism showed that (1) toxaphene did not alter the growth rate of MCF-7 cell cultures over 13 d, and (2) toxaphene did not alter the catabolism of E2. In conclusion, results from the MCF-7 focus assay demonstrate that toxaphene is weakly antiestrogenic rather than estrogenic.