16 alpha-Hydroxyestrone, a product of estrogen 16 alpha-hydroxylation in humans that is suspected to be implicated in cell transformation, has been found to form stable adducts with nuclear components. The stable covalent adduct formed from 16 alpha-hydroxyestrone with 2-methoxyethylamine via the Heyns rearrangement of the alpha-hydroxyimine was identified as 3-hydroxy-17 beta-(2-methoxyethylamino)estra-1,3,5(10)-trien-16-one. Since the same product was obtained from 16 beta-hydroxyestrone with the amine, the alpha-hydroxyenamine is the most likely intermediate of the Heyns rearrangement. The adduct was fairly stable at 37 C in phosphate buffer (pH 7.4)/methanol (1:1 v/v), while the adduct formed from 16-oxoestradiol was disrupted reversely and completely within 6 hours. The evidence suggests that N-(3-hydroxy-16-oxoestra-1,3,5(10-trien-17 beta-yl)amine is the partial structure of the stable adducts formed from D-ring alpha-ketol estrogens with proteins.