The early destruction of grafted or transplanted material, usually beginning a week after implantation. Acute rejection is identified clinically by decreased function of the transplanted organ. High-dose corticosteroids are the first treatment of acute rejection; they are typically quite effective. Antilymphocyte globulin (ALG), the monoclonal antibody OKT 3, mycophenolate mofetil, and tacrolimus, among other agents, are used when corticosteroids are not effective.