Circulating inhibitors of granulopoiesis in patients with aplastic anaemia.

An inhibitor of granulopoiesis has been detected in the serum of aplastic patients by its ability to reduce colony formation by normal human marrow cells in culture. The inhibitory activity in the serum was abolished when a patient was treated with azathioprine and exposure of aplastic marrow to antithymocyte globulin (ATG) in vitro released the colony forming cells from suppression. The clinical findings in three of the patients studied support the idea that the inhibitory activity detected in vitro is associated with normal marrow suppression in vivo.