A purified gamma immunoglobulin (IgG) with immunosuppressive activity. Obtained from rabbits that have been immunized with human thymocytes, antithymocyte globulin specifically recognizes and destroys T lymphocytes. Although the exact mechanism of action is not completely understood, it appears to involve T lymphocyte clearance from the circulation and modulation of T lymphocyte activity. Administering antithymocyte globulin with chemotherapy prior to stem cell transplantation may reduce the risk of graft-versus-host disease.