A differentiation-inducing factor (DIF) for the promyelocytic HL-60 cell line is constitutively produced by the malignant T lymphocyte line HUT-102. DIF was highly purified from HUT-102-conditioned media by means of diethylaminoethanol (DEAE)-chromatography, gel chromatography, and high-resolution, ion-exchange chromatography on a MonoQ column and sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). In addition to inducing differentiation of wild-type HL-60 cells, resulting in secondary inhibition of growth, DIF, at a tenfold lower concentration, inhibited the growth of some clones of the monoblastic U-937 cell line as well as that of subclones of HL-60. The latter effect was most likely a primary growth inhibition and not secondary to differentiation; 50% inhibition of clonogenic growth in agar was seen at approximately 1.0 pmol/L of DIF. In addition, the clonogenic growth of fresh leukemia cells from 10 of 12 patients with acute myeloid leukemia (AML) was inhibited with 50% inhibition at approximately 10 pmol/L of DIF. The growth of normal granulocyte-macrophage colonies was inhibited at a similar concentration, whereas early erythroid colonies were much more resistant. DIF and interferon-gamma (gamma-IFN) were shown to be separate molecules inasmuch as a neutralizing antibody for gamma-IFN did not abolish the DIF effect. The differentiation effect on wild-type HL-60 and the proliferation inhibitory effect on leukemic and normal myeloid cells cochromatographed through all purification steps suggest that both activities are exhibited by identical polypeptides. DIF may have a role in regulating normal hemopoiesis. The growth inhibitory effect of DIF and the ability to induce differentiation of some leukemia cells may suggest a clinical utility in the treatment of leukemia.