Anti-thymocyte globulin (ATG) is an infusion of horse or rabbit-derived antibodies against human T cells and their precursors (thymocytes), which is used in the prevention and treatment of acute rejection in organ transplantation and therapy of aplastic anemia due to bone marrow insufficiency.

rATG in particular effects large reductions (through cell lysis) in the number of circulating T lymphocytes, hence preventing (or at least delaying) the cellular rejection of transplanted organs.

[citation needed] ATG use can induce cytokine release syndrome, and has been thought to increase the risk of post-transplant lymphoproliferative disorder (PTLD); however, this association may not apply when lower dosing regimens are used.

Anti-IL-2Rα receptor antibodies such as basiliximab and daclizumab are increasingly being used in place of ATG as an induction therapy, as they do not cause cytokine release syndrome and (theoretically) improve the development of tolerance.

[4] The Canadian Blood and Marrow Transplant Group is currently conducting the first randomised trial in chronic graft versus host using an even lower dose of rabbit ATG (4.5 mg/kg) in an attempt to confirm these observations.