Thirty-six patients with aplastic anemia and three with aplastic anemia-PNH syndrome in eight institutes were studied for the presence of peripheral blood mononuclear cells having a suppressive effect on granulocyte-macrophage (G-M) and erythroid colony formation using a uniform protocol. In 11 cases of 29 (38%) and 9 cases of 29 (31%), the presence of mononuclear cells with a suppressive effect on G-M and erythroid colony formation, respectively, was demonstrated. However, the presence of mononuclear cells suppressive both to G-M and erythroid colony formation was demonstrated only in 3 of 19 cases (16%).