Microvascular endothelial cell cultures have been established from mouse lung, liver, brain, heart, placenta, kidney, urinary bladder, mammary gland, ovary and epididymal fat pad. In addition, large vessel endothelial cells have been obtained from the mouse aorta and thoracic duct. The heterogeneity of these cells has been shown by flow cytometric determination of angiotensin-converting enzyme, by differential presence of the acetyl low density lipoprotein receptor, by the variable expression of cell surface antigens, and by differential binding of various plant lectins. The endothelial cell lines we have developed provide the means to examine in the mouse, long a key species for biomedical research, a wide range of biological functions and properties of the vascular endothelium.