Leukaemia inhibitory factor (LIF) is a cytokine that induces macrophage differentiation of the murine M1 myeloid leukaemia cell line. We have isolated a cDNA clone encoding a novel human haemopoietic growth factor, human interleukin for DA cells (HILDA) that supports the proliferation of the murine interleukin-3-dependent leukaemic cell line, DA-la (refs 3-5). HILDA proved to be identical to LIF. The demonstration that the differentiation factor LIF will also serve as a growth factor for at least one myeloid leukaemic cell line provides further evidence that the distinction between growth-promoting and differentiation-inducing activities are largely determined by the target cell type.