AC133 is one of a new panel of murine hybridoma lines producing monoclonal IgG antibodies (mAbs) to a novel stem cell glycoprotein antigen with a molecular weight of 120 kD. AC133 antigen is selectively expressed on CD34bright hematopoietic stem and progenitor cells (progenitors) derived from human fetal liver and bone marrow, and blood. It is not detectable on other blood cells, cultured human umbilical vein endothelial cells (HUVECs), fibroblast cell lines, or the myeloid leukemia cell line KG1a by standard flow cytometric procedures. All of the noncommitted CD34+ cell population, as well as the majority of CD34+ cells committed to the granulocytic/monocytic pathway, are stained with AC133 antibody. In vitro clonogenicity assays have demonstrated that the CD34+AC133+ double-positive population from adult bone marrow contains the majority of the CFU-GM, a proportion of the CFU-Mix, and a minor population of BFU-E. The CD34dim and AC133− population has been shown to contain the remaining progenitor cells. AC133-selected cells engraft successfully in a fetal sheep transplantation model, and human cells harvested from chimeric fetal sheep bone marrow have been shown to successfully engraft secondary recipients, providing evidence for the long-term repopulating potential of AC133+ cells. A cDNA coding for AC133 antigen has been isolated, which codes for a polypeptide consisting of 865 amino acids (aa) with a predicted size of 97 kD. This antigen is modeled as a 5-transmembrane molecule, a structure that is novel among known cell surface structures. AC133 antibody provides an alternative to CD34 for the selection and characterization of cells necessary for both short- and long-term engraftment, in transplant situations, for studies of ex vivo expansion strategies, and for gene therapy.