There is a microcirculation system within the islets of Langerhans. However, little is known about the phenotypicand functional characterization of islet microvascular endothelial cells (MVEC). In this study, we purified MVEC from human pancreatic islets by using Ulex europaeus(Sigma, St. Louis, MO)agglutinin-1(UEA-1)-coated dynabeads(Dynal A. S., Oslo, Norway). These purified human islet MVEC (HI-MVEC) express von Willebrand factor, take up high levels of acetylated LDL, and upregulate endothelial cell leukocyte adhesion molecule 1 in response to tumor necrosis factor-a. Ultrastructure examination shows the presence of microvilli and fenestrations on the cell surface, Weibel-Palade bodies in the cytoplasm, and tight junctions between cells. Furthermore, we show that vascular endothelial cell growth factor contributes to the formation of surface fenestrations on cultured HI-MVEC. After purification, HI-MVEC exhibit a very low proliferation capacity and are strongly resistant to trypsin, compared with other original MVEC. We also demonstrate that a-1 proteinase inhibitor(Api)is expressed on HI-MVEC and specifically located at the area of cell-cell junctions. By reverse transcription-polymerase chain reaction, a significant messenger RNA band of Api was found only in HI-MVEC, but not in other organ-derived MVEC, indicating that expression of Api is islet MVEC specific. Antibodies to Api significantly reversed the resistance to trypsin and promoted proliferation of HI-MVEC, suggesting that these specific functional characteristics of HI-MVEC are related to the expression of Api. These results indicate that HI-MVEC exhibit some specific morphological and functional characteristics that differ from MVEC derived from other organs. Diabetes 48: 1773-1778, 1999