Microvasculature plays an important role in a variety of physiological and pathological processes. The authors have previously shown that primary cultures of human microvascular endothelial cells consist of 2 distinct populations of blood vascular and lymphatic endothelial cells. These subtypes are ephemeral and lose purity through passaging. To generate reproducible in vitroand in vivoexperiments stable blood and lymphatic endothelial cell lines are an essential prerequisite. In this study they have used human telomerase gene-immortalized nontransformed human microvascular endothelial cell cloned pure cultures of blood and lymphatic endothelial cell subpopulations. Flow cytometry, immunofluorescence, Northern and Western blotting, microarray gene analysis, as well as basic functional assays were used to characterize these clones. Immortalized blood and lymphatic subpopulations are stable and functionally specialized cell lineages that expressed pan-endothelial and cell-type-specific markers. They are excellent candidates for long-term culture studies on microvascular-related diseases. Microcirculation(2004) 11,261–269. doi:10.1080/10739680490425967