FR-900490 is a new type of immunoactive substance produced by a fungus Discosia sp. F-11809. The colony forming units in culture (cfu-c) in bone marrow cells, which were suppressed by immunosuppressive factor obtained from the serum of sarcoma 180 tumor bearing mouse, was restored to normal level by the addition of FR-900490 in vitro. Furthermore, in mitomycin C (MMC)-treated mice the subsequent administration of FR-900490 caused a significant increase of cfu-c in bone marrow cells depressed by MMC.