A novel human cell line, KMT-2, from umbilical cord blood cells was established based on the selection of cultures in the presence of recombinant human interleukin-3 (IL-3) and the sorting of cells with anti-My 10 antibody. Morphologic and cytochemical studies (peroxidase negative, Sudan-black negative, chloroacetate esterase negative, PAS positive, nonspecific esterase positive) and phenotyping (HLA-DR, My7 = CD13, My9 = CD33, My10 = CD34, MCS-2, LeuM1 positive, glycophorin A negative, and P2 negative) suggest that the KMT-2 cells are myelomonocytic cells, probably of immature progenitor origin. Besides IL-3, granulocyte-macrophage colony-stimulating factor supported the growth of the KMT-2 cells, but IL-1 alpha, IL-2, IL-4, IL-5, and erythropoietin did not. IL-6 showed only slight activity. Binding studies with 125I-labeled recombinant human (rh) IL-3 indicated that IL- 3 bound to a single class of high affinity receptors (approximately 4,000 receptors/cell) on KMT-2 cells with a kd of approximately 200 pmol/L. The chemical cross-linking assay demonstrated that radiolabeled hIL-3 bound three molecules with molecular masses of 170, 130, and 70 Kd. Present data suggest that the newly established human cell line will be a valuable tool for the biologic assay of hIL-3, and a model for biochemical studies of IL-3 receptors.