Interleukin-2 (IL-2) stimulated H-thymidine incorporation in the blasts of six of 21 cases of acute myeloid leukaemia (AML). An IL-2 induced increase in cell numbers was directly demonstrated in the two patients studied in this way, and T-cell contamination was rigorously excluded. The IL-2-induced proliferation was usually less marked than that caused by granulocyte-macrophage colony stimulating factor (GM-CSF), and IL-2 moderately enhanced GM-CSF-induced stimulation in five of the six patients; in the sixth, IL-2 and GM-CSF were strongly synergistic. IL-2-induced proliferation was observed only in AML with a monocytic component (M4/M5), but not all M4/M5 leukaemias responded to IL-2. There was no correlation between expression of the light-chain of the IL-2 receptor and IL-2-induced stimulation. It is suggested that IL-2 is involved at a restricted stage of early myelopoiesis, perhaps when cells are becoming committed to the monocytic lineage; and that IL-2 is a growth factor for early myeloid cells in a proportion of cases of AML.