Leukaemia inhibitory factor (LIF) can induce macrophage differentiation in M1 murine myeloid leukaemic cells and suppress their proliferation in vitro. It does not stimulate the proliferation of normal progenitor cells and is apparently distinct from known colony-stimulating factors. We have used oligo-nucleotides complementary to partial amino acid sequence of LIF to isolate a LIF clone from a T lymphocyte cDNA library. When this cDNA was coupled to a yeast expression vector (YEpsec1) and introduced into yeast cells, a molecule with the biological properties characteristic of native LIF was secreted into the growth medium. The amino acid sequence of LIF established it to be a unique molecular entity, distinct from the other known haemopoietic growth factors. Since LIF is encoded by a unique gene, two biochemically separable forms of LIF probably represent post-transcriptional or posttranslational variants of the same gene product. In contrast to several other haemopoietic regulators, the 0.8- to 1-kb LIF mRNA was expressed constitutively in two murine T lymphocyte cell lines examined, and its abundance was not enhanced by stimulation with concanavalin A. Cloning, sequencing and expressing LIF has resolved several discrepancies in the literature concerning the identity of factors capable of inducing differentiation of murine myeloid leukaemic cells in vitro.