A receptor tyrosine kinase (RTK), TIE (tyrosine kinase that contains immunoglobulin-like loops and epidermal growth factor [EGF] homology domains), is expressed in vascular endothelial and hematopoietic cells. We generated monoclonal antibodies (MoAbs) against the extracellular domain of TIE and a polyclonal antibody against the TIE carboxyterminus and used them to analyze expression of TIE in hematopoietic cells. Western blotting detected two forms of TIE protein with a molecular mass of 135 and 130 kD in hematopoietic and endothelial cells. Northern blotting analysis revealed that TIE was expressed preferentially in undifferentiated cell lines, especially when megakaryocytic, but not erythroid differentiation was induced. Reverse transcriptase-polymerase chain reaction (RT-PCR) showed that TIE was predominantly expressed in the human hematopoietic progenitor fraction, CD34+ cells. Fluorescence- activated cell sorting (FACS) showed that 42% of CD34+ and 17% of KIT- positive (KIT+) cells were TIE-positive (TIE+). The majority (81%) of the primitive hematopoietic stem cells, CD34+CD38- cells, were TIE+. Assays of progenitor cells and long-term culture-initiating cells (LTC- IC) showed that the TIE+ fraction contained more primitive cells than the TIE- fraction. Some TIE+ cells were in the CD34- fraction, which were CD19+ and CD20+ (B cells). These findings indicate that TIE has a unique spectrum of expression in primitive hematopoietic stem cells and B cells. Although its ligand has not been identified, TIE and its ligand may establish a novel regulatory pathway not only in early hematopoiesis, but also in the differentiation and/or proliferation of B cells.