The present paper reports the culture of human microvascular endothelial cells under improved culture conditions. The cells were isolated from omental tissue obtained at abdominal surgery by Kern's method and were cultured in non-coated tissue culture dishes in either TC199 or MCDB131 medium supplemented with fetal bovine serum. The cultured cells were identified as endothelial cells by immunofluorescence for Factor VIII-related antigen and by the ability to take up a acetylated LDL and to produce prostacyclin. Since the production of prostacyclin decreases with the number of transfers, it is desirable to use the cells from the second to the fourth transfer generations for the experiments. The cells were found to have the insulin receptor, and the number of receptors was calculated to be 104 per cell. Growth and maintenance of the cells were better in the cells cultured with MCDB131 medium than with TCM199. This culture system provides a large number of pure human microvascular endothelial cells and is of great value for studying the pathogenesis of diabetic microangiopathy.