Of a large number of steroid hormones, semisynthetic analogues, and metabolites examined for activity in an opiate radioreceptor binding assay, only 17 alpha -estradiol (E-17 alpha) and closely related 17 alpha-dihydroequilin (DHE1) and 17 alpha-dihydroequilenin (DHE2) were active. 17 beta-estradiol (E-17 beta) was much weaker than E-17 alpha, particularly in the presence of Na+. On the basis of the effects of Na+ and Mn+2 on binding potency, E-17 alpha is an agonist/antagonist and DHE1 and DHE2 pure antagonists. There is evidence for diverse biological activities of 17-alpha estranes, some of which may be mediated through opiate receptors.