PURPOSE: To measure more easily and accurately estradiol in a blood or urine compared with conventional methods, by using an antibody of a new antigen prepared by a specified method.

CONSTITUTION: An antigen, shown in a formulaI(NH-BSA is a bovine serum albumin residue, G is a glucronyl group shown in a formula II) is prepared. This antigen is obtained by introducing a glucronyl group shown in a formula IV into OH of 6-oxo-estradiol-3-benzylether shown in a formula III not to combine the BSA with glucronic acid part, and bonding NHCH2COOH to C6 position of B ring, and treating with bovine serum albumin, then hydrolyzing the acetyl group of the glucronyl group to OH. By using the antiserum obtained by injection of this antigen to a rabbit, estradiol in a blood or urine can be determined easily and accurately by an immunoassay method, particularly radioimmunoassay, without any necessity of hydrolyzing the blood or urine to liberate the estradiol.