Although multiple culture assays have been designed to identify endothelial progenitor cells (EPCs), the phenotype of cells grown in culture often remains undefined. We sought to define and characterize the proangiogenic cell population within human peripheral blood mononuclear cells. Mononuclear cells were isolated from peripheral blood and grown under angiogenic conditions for 7 days. Formed colonies (CFU-As) were identified and analyzed for proliferation, mRNA and surface antigen expression, tube-forming ability, and chromosomal content. Colonies were composed of a heterogeneous group of cells expressing the leukocyte antigens CD45, CD14, and CD3, as well as the endothelial proteins vascular endothelial (VE) cadherin, von Willebrand's factor (vWF), CD31, and endothelial nitric oxide synthase (eNOS). Colony cells expressed increased levels of proangiogenic growth factors, and they formed tubes in Matrigel. In comparison with colonies from the CFU-Hill assay, our assay resulted in a greater number of colonies (19 ± 9 vs. 13 ± 7; p < 0.0001) with a substantial number of cells expressing an endothelial phenotype (20.2% ± 7.4% vs. 2.2% ± 1.2% expressing eNOS, p = 0.0006). Chromosomal analysis indicated the colony cells were bone marrow derived. We, therefore, describe a colony-forming unit assay that measures bone marrow-derived circulating mononuclear cells with the capacity to proliferate and mature into proangiogenic leukocytic and endothelial-like cells. This assay, therefore, reflects circulating, bone marrow-derived proangiogenic activity.