We have studied the marrow cells from a patient with acute myeloid leukemia (AML) for their responsiveness to colony-stimulating activity (CSA) in vitro. The AML cells were stimulated by CSA to rapid and extended growth in liquid culture. In the absence of CSA, the majority of cells died. CSA also stimulated the clonal growth of AML cells, and the minimum requirement for CSA was one-tenth to one-fiftieth that required to stimulate the growth of normal marrow CFU-C. CSA for AML cells was eluted from Sephacryl S-200 columns in fractions that represented an apparent molecular weight of 45,000 daltons. This fraction also produced optimal stimulation of normal human marrow. During remission, the patient's marrow cells did not grow in liquid culture and produced normal numbers of granulocytic and erythroid colonies in response to CSA and erythropoietin. Extended culture of the AML cells resulted in cell differentiation evidenced by decreasing proliferative capacity and by morphological and histochemical changes. These studies indicate that certain AML cells are extraordinarily responsive to CSA, an in vitro mediator of normal granulopoiesis.