Recent findings on vertebrate embryos have provided compelling evidence for the existence of hemangioblasts, i.e. common precursors for endothelial and hematopoietic cells, characterized by expression of the VEGFR2/Flk1 receptor. We describe here a population of KDR+ CD34(-) mesoderm cells that emerges in early-semitic human embryos, by the beginning of the 4th week of gestation. In the developing blood vessels, KDR-expressing CD34(-) cells gradually coexpress increasing levels of CD34 antigen. Remarkably, as development proceeds, a KDR+ CD34(-) contingent persists in the paraaortic splanchnopleura until just prior to the emergence of aorta-associated hematopoietic cell clusters. These observations suggest that KDR+ CD34(-) mesodermal cells might represent the putative hemangioblastic precursor of human hematopoietic and endothelial lineages. (C) 1999 Elsevier Science Ireland Ltd. All rights reserved.