The large capillary mass of the newborn lung demands the presence of endothelial cell precursors in lung tissue prior to development of the pulmonary capillary bed. The objective of this investigation was to isolate and characterize putative endothelial cell precursors from developing human lung. CD34, a cell surface marker for hematopoietic progenitor cells, endothelial precursor cells and small vessel endothelial cells was employed as an immunological ''handle'' for the selection of the desired cells. When CD34+ cells were isolated from midtrimester human fetal lung tissue then maintained in culture, the isolated cells expressed immunoreactivity for the endothelial cell marker von Willibrand's factor, and the vascular endothelial growth factor receptors, KDR and Flt-1. However, only 5% or fewer of the cells expressed PECAM an important factor in cell - cell interactions and a marker for endothelial cells associated with vessels. The CD34+ cells endocytosed acetylated low-density lipoprotein and formed capillary- like structures when incubated in a cushion of Matrigel. RT-PCR analysis of mRNA for endothelial cell related proteins Flt-1, Tie-2 and eNOS demonstrated expression of these mRNAs by the isolated cells for at least 16 cell passages. These observations demonstrate that capillary endothelial precursor cells can be isolated from developing human lung and maintained in cell culture. These cells represent a potentially important tool for investigating the regulation of mechanisms governing development of the air-blood barrier in the human lung.