A new cell line was established from the bone marrow of a patient with chronic myeloid leukemia. The cells were attributed an intermediate myeloid phenotype on the basis of their cytochemical features and membrane antigen expression. These cells respond to both chemical and physiological activators of the signal transduction pathways with growth arrest and phenotype changes. Macrophage maturation can be induced in a fraction of the cells by gamma-interferon (γ-IFN). Cells are however recruited again into the cell cycle by recultivation in γ-IFN-free medium: variants unresponsive to γ-IFN, and others which show either reversible or irreversible differentiation were isolated from the original cell line by cloning and sib-selection. These clones can be used to investigate the relationship between γ-IFN response pathways and cell proliferation.