Laboratory grade methoxychlor (99% pure), base-washed methoxychlor, and a metabolite of methoxychlor, 2,2-bis(p-hydroxyphenyl)-1,1,1-trichloroethane (HPTE), were tested for their ability to compete with [3 H] estradiol-17 beta ([3 H]E2) for specific binding to the estrogen receptor from immature rat uterine cytosol. The binding was determined on 10--30% sucrose gradients and by a dextran-coated charcoal assay and subsequent Scatchard plot analysis. On gradients, laboratory grade methoxychlor, but not base-washed methoxychlor, suppressed [3 H]E2 binding to the 8S estrogen receptor. However, the base-soluble fraction from washing of laboratory grade methoxychlor caused suppression o[3 H]E2 binding on sucrose gradients at a concentration as low as 3.6 ppm. Scatchard plot analysis indicated that the inhibition of binding observed with laboratory grade methoxychlor was competitive in nature and not caused by receptor destruction. It was concluded that laboratory grade methoxychlor contained a contaminant that was potentially estrogenic. HPTE, an in vivo metabolite of methoxychlor, caused a marked suppression of [3 H]E2 binding in the 85 region of the gradients. Analysis by Scatchard plot indicated that the effect of HPTE was not to decrease the number of E2 binding sites but merely to alter the affinity of binding to the receptor, presumably in a competitive manner. The low K1 value for HPTE suggested an extremely high affinity for uterine cytosolic E2 receptors.