A subset of circulating CD34+ cells was found to express CD105 antigen. Sorting experiments showed that most granulocyte-macrophage colony-forming units (GM-CFU) and burst-forming units - erythroid (BFU-E) were retained in the CD34+/CD105- fraction, whereas rare GM-CFU/BFU-E were generated from CD34+/CD105+ cells. Megakaryocytic aggregates were entirely retained in the CD34+/CD105+ fraction. Neutralizing doses of an anti-TGF-beta1 antibody demonstrated CD34+/CD105+ cells capable of colony-forming activity without any significant effect on CD34+/CD105- cells. Cloning of secondary colonies revealed that CD34+/CD105+ cells had a significantly higher secondary cloning efficiency than CD34+/CD105- cells. CD34+/CD105+ cells had a significantly higher long-term culture-initiating cell (LTC-IC) frequency than CD34+/CD105- cells. Kinetic analysis showed that 75% of CD34+/CD105+ cells consisted of DNA 2n G0Ki-67- cells whereas 82% of CD34+/CD105- were DNA 2n G1Ki-67+ cells, and this latter subset showed a RNA content consistently higher than CD34+/CD105+ cells. CD34+/CD105+ progenitors were CD25+, whereas CD34+/CD105- contained a small CD25+ subset. Three-colour analysis of bone marrow and cord blood CD34+ cells demonstrated that all the CD34+/CD38low/- primitive precursors were contained in CD34+/CD105+ cells. Extensive characterization of these CD105+ precursors indicated that they have biological properties associated with primitive haematopoietic precursors.