Isogarcinol is a natural polyisoprenylated benzophenone first isolated from plant species in the genus Garcinia. It has immunosuppressant actions, inhibiting the protein phosphatase calcineurin (IC50 = 36 µM) and suppressing the proliferation of T cells. Oral administration of isogarcinol in mice decreases delayed type hypersensitivity, prolongs graft survival in allogeneic skin transplants, suppresses inflammation in collagen-induced arthritis, and reduces clinical symptoms in experimental autoimmune encephalomyelitis. Isogarcinol inhibits the proliferation of HL-60 and PC-3 cancer cells (IC50s = 4 and 8 µg/ml, respectively) through cell cycle arrest and apoptosis.