Acetylated low density lipoprotein is metabolized by a receptor-mediated process in endothelial cells. We have used the lipoprotein labelled with the fluorescent probe 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate to localize endothelial cells lining blood vessels. Following intravenous injection of the labelled lipoprotein, the vascular sinusoids and all other hepatic blood vessels were clearly labelled in cryostat sections of mouse liver. The endothelium of other organs such as brain, kidney, and testis was also brightly labelled. In addition, the lipoprotein was used to label the endothelium of bovine aorta, the vasculature in the chick chorioallantoic membrane, and the vessels in a growing murine melanoma. These results demonstrate that the fluorescent-labelled lipoprotein can be used for in situ labelling of the endothelium from large as well as small blood vessels in a variety of species.