The syntheses and estrogen receptor affinities of meso- and (+/-)-N,N'-dialkyl-1,2-bis(2,6-dichloro-4-hydroxypheny)ethylenediamines (2) are described. They show high binding affinities in both diastereomeric forms but with a preference for the meso isomer, reaching a RBA value of 8.6 (meso-2b; 17 beta-estradiol = 100). Both stereoisomers of 2b exhibit a strong inhibitory effect on the 7,12-dimethylbenz[alpha]anthracene (DMBA) induced hormone-dependent mammary carcinoma of the Sprague-Dawley rat, reducing the tumor area by 74 (meso-2b) and 24% [(+/-)-2b], respectively, after 4 weeks administration of 6 x 6 (mg/kg)/week. The high uterotrophic potency makes a mode of action likely which corresponds to the effect of high doses of estrogens.