Certain antiuterotrophic agents, such as ethamoxytriphetol, nafoxidine, clomiphene and Parke-Davis CI-628, act by preventing the association of estrogenic hormones with the extranuclear receptor protein of target tissues. These agents have proved of considerable value in estrogen receptor studies by permitting the distinction between specific hormone-receptor interaction and artifacts of nonspecific binding. Examples include evidence that estrogen-receptor interaction plays a role in uterine growth induction, the first indication of a relation between cytosol and nuclear binding, the relation between steroid structure and specific uterine binding invitro and the prediction of response of human breast cancers to adrenalectomy orhypophysectomy.