Clinical responses of patients with acute leukemia (AL) following the myelodysplastic syndrome (MDS) to the standard therapy are poor. It is considered that the greatly decreased hemopoiesis in these cases is responsible for their clinical picture. We studied leukemia-associated inhibitory activity (LIA), which inhibited human granulocyte-macrophage progenitors, in these patients. Peripheral blood (PB) mononuclear cells (MNC) suppressive to granulocyte-macrophage colony formation were present in 3 of 5 cases of de novo AL and in 3 of 4 cases of AL following MDS. The PB MNC-cultured media from these cases also suppressed colony formation. The elution patterns of LIA of these cases were almost identical in gel chromatography. These results suggest that LIA may be responsible for the suppression of normal granulopoiesis in some patients with AL developed from MDS, and that the profound derangement of normal hemopoietic capability in these cases may be due to multiple complex factors.