For the purpose of obtaining antisera for radioimmunoassay of 2- and 4-hydroxyestradiol, four haptens possessing a carboxyl group have been synthesized. 6-Oxo-2-hydroxyestradiol and 6-oxo-4-hydroxyestradiol were synthesized from estradiol in several steps. The condensation of 6-oxo catechol estrogens with O-carboxymethylhydroxylamine provided the desired 6-(O-carboxymethyl) oximes as new haptens. The preparation of 2- and 4-hydroxyestradiol 17-hemisuccinates as another type of hapten was also carried out by using succinic anhydride and pyridine.