Cells derived from omental fat tissue and used for seeding vascular prostheses are not endothelial in origin. A study on the origin of epitheloid cells derived from omentum. The use of microvascular endothelial cells derived from omental tissue has been advocated to seed vascular grafts with autologous endothelial cells in high density. The purpose of our study was to evaluate the precise origin of these cells. Therefore we have compared cellular characteristics of these cells with those of endothelial cells isolated by collagenase treatment of human umbilical veins. The omental cells were isolated from from omental tissue from four different patients by incubation in a collagenase-dispase solution. Part of the material was processed by Percoll density gradient centrifugation in an attempt to purify the isolates. Cellular characteristics of both types of cells were determined by studying the morphologic features of the cells and by determining the presence of von Willebrand factor, antigens EN-4 and PAL-E specific for endothelial cells, cytokeratins 8 and 18, vimentin and desmin, and uptake of diI-acetylated low-density lipoprotein. Epitheloid cells from omental tissue, isolated after collagenase treatment and either purified or nonpurified by Percoll density gradient centrifugation, differed from human umbilical vein endothelial cells with respect to the presence of surface microvilli, the expression of von Willebrand factor, EN-4 and PAL-E, and the presence of cytokeratins 8 and 18 and desmin. von Willebrand factor (in a granular staining pattern) and the presence of EN-4 and PAL-E were only detected in human umbilical vein endothelial cells. Vimentin was present in both cell types, whereas cytokeratins 8 and 18 and desmin were only present in cells derived from omentum. From these data we conclude that the so called microvascular endothelial cells from omentum are not endothelial but mesothelial in nature. 