Tumor necrosis factor alpha (TNF-alpha) and gamma-interferon (IFN-gamma) have been shown to suppress clonogenic growth in cultures containing blast cells obtained from patients with acute myeloid leukemia. We report that recombinant human TNF-alpha and IFN-gamma are also able to induce functional and morphological maturation in fresh myeloid leukemic cells in vitro. Assessing suspension cultures containing cells from patients with acute myeloid leukemia (11 patients) or myeloid blast crisis of chronic myeloid leukemia (5 patients), it was found that recombinant human TNF-alpha and IFN-gamma significantly enhanced the number of cells reducing nitroblue tetrazolium, as compared to control cultures containing no cytokine (P less than 0.001 and P less than 0.001, respectively). Cells from responders showed alterations characteristic of monocyte/macrophage differentiation, adherence to plastic surfaces, development of positive staining for alpha-naphthyl acetate esterase, typical morphology, and expression of cell surface antigens detected by the monoclonal antibodies Mo-1, Mo-2, and My-4. Both cytokines decreased the number of viable cells, the number of blast cells, and the number of cluster-forming units in suspension culture, suggesting inhibitory actions on the growth capacity of leukemic cells. Compared to the maximum effects of either factor alone, the combination of recombinant human TNF-alpha and IFN-gamma significantly increased the extent of growth inhibition and cell adherence but did not result in further increases in nitroblue tetrazolium reduction. The presence of Auer rods in IFN-gamma or TNF-alpha differentiation-induced macrophages with cells from a patient with M5 acute myeloid leukemia demonstrates that these cytokines can induce differentiation of a leukemic clone in primary cells from patients with leukemia.