The monoblast-like human histiocytic lymphoma cell line U937 can be induced by phorbol 12-myristate 13-acetate (PMA) at concentrations of 8-16 ng/ml to undergo differentiation. The induced cells show growth inhibition, morphological changes to monocyte-macrophage-like cells, and increases in nitroblue tetrazolium (NBT) and nonspecific esterase activities. The expression of surface markers MO1 and MO2 for macrophages is also elevated. The induced differentiation of U937 cells is accompanied by the appearance of colony-stimulating activity in the culture medium as assayed on mouse bone marrow cells. Gel filtration chromatography of the conditioned media shows a peak of colony-stimulating factor (CSF) activity with an apparent molecular weight of approximately 100 kd. Morphological analysis of the colonies reveals a predominant monocyte-macrophage population. This CSF activity can be neutralized by anti-human colony-stimulating factor 1 (CSF-1) antibody, and Western blot analysis shows a CSF-1 band. All these results strongly indicate that CSF produced by induced U937 cells is CSF-1. Northern blot analysis of RNA isolated from U937 cells demonstrated that CSF-1 gene expression started after PMA induction for 3 h and declined after 18 h. Protooncogenes c-fos and c-jun were expressed after half an hour of incubation with PMA.