PURPOSE: To obtain an immunosuppressive agent, containing an antiesteradiol antibody as an active ingredient and useful for treating rejection of organ transplanting and autoimmune diseases without any side effects.

CONSTITUTION: An suppressive agent is obtained by including an antiestradiol antibody, e.g. an antiesteradiol monoclonal antibody prepared by binding estradiol to carboxymethyl oxime, etc., using the resultant estradiol as a hapten, binding the hapten to bovine blood serum albumin, etc., preparing an estradiol- bovine blood serum albumin, etc., immunizing an animal such as a rat therewith or fusing the obtained antiserum or splenic cells from the immunized animal to myelomatous cells and releasing the antiestradiol monoclonal antibody from cloned cells of the resultant hybridoma as an active ingredient. The aforementioned immunosuppressive agent is useful also for enhancing immunosuppressive action on an immunosuppressive agent (e.g. cyclosporin or rapamycin) having suppressing action on lymphocyte blast transformation reaction with currently used concanavalin A, etc., as lectin.