Endothelial cells alter morphology easily when culture conditions change. One has omitted to test carefully whether this heteromorphology represents constitutional differences which are maintained in culture. Cells were isolated from the microvascular tree of the bovine corpus luteum by mechanical disintegration, i.e. without collagenase treatment. From these cells, 5 phenotypes were purified which formed epithelial monolayers of stable morphology in long-term culture. Each phenotype showed criteria typical of endothelial cells. The criteria were incompletely present in phenotype 5. Differences between the phenotypes related to the structure of the cytoskeleton, the fibronectin matrix and to the expression of cell-cell adhesion molecules from the cadherin family and the NCAM (neuronal cell adhesion molecule) group. Mesothelial cells, pericytes, myocytes as well as luteal cells were considered for differential diagnosis. The present results point to a heterogeneous distribution pattern of endothelial cells within the microvascular tree of a single organ.