Using a fragment of the murine flt3 ligand as a probe, we have succeeded in cloning a human flt3 ligand from a human T-cell lambda gt10 cDNA library. The human and murine ligands are 72% identical at the amino acid level. Analysis of multiple cDNA clones shows that alternative splicing of the human flt3 mRNA can occur at a number of positions. A recombinant soluble form of the human flt3 ligand stimulates the proliferation and colony formation of a subpopulation of human bone marrow cells that are CD34+ and are enriched for primitive hematopoietic cells. In addition, the human flt3 ligand also stimulates the proliferation of cells expressing murine flt3 receptors. Northern blot analysis shows widespread expression of flt3 ligand mRNA transcripts in human tissues.