We have generated mutant mouse estrogen receptors which differ in their sensitivity to estrogen and the antiestrogen 4-hydroxytamoxifen. Mutation of the glycine at position 525 and the methionine and/or serine at positions 521/522 virtually abolishes the ability of the receptor to bind estradiol and stimulate transcription. In contrast, the mutant receptors retain the partial agonist activity exhibited by the wild-type receptor in the presence of 4-hydroxytamoxifen. The mutations do not affect the expression and DNA-binding activity of the receptor, but do abolish the estrogen-induced increase in the mobility of the receptor-DNA complex observed with the wild-type receptor. Other mutant receptors that were able to bind and stimulate transcription in the presence of estradiol also failed to show the agonist-induced increase in the mobility of the receptor-DNA complex, suggesting that it is unlikely to reflect the formation of a hormone-dependent transcriptional activation function.