To describe the suppressive effect of sera from patients with aplastic anemia (AA) on heamatopoietic progenitor cells and explore further the features of serum inhibitor, we first examined the inhibiting activity of the serum on the colony formation of burst-forming units of erythrocyte (BFU-E) and colony-forming units of granulocyte/macrophage (CFU-GM). The potent inhibiting activity was demonstrated in six of 20 patients. Five of the six patients were severe AA (SAA). The suppressive effect did not change after elimination of monocyles and lymphocytes from target MNC. The haematopoietic inhibitory activity of sera from three of the six patients was almost totally neutralized by the MoAb against gamma-IFN. All these three patients were SAA. These data suggest that inhibitor of haematopoietic progenitor cells is present in most of SAA patients. In some patients, the inhibitor was gamma-IFN, which play an important role in the pathogenesis of SAA.