Isolated rat brain capillaries were analyzed by confocal microscopy. Fluorescent immunoliposomes bearing the OX26 anti-transferrin receptor monoclonal antibody were synthesized and incubated with freshly isolated unfixed microvessels to visualize binding to luminal and abluminal membranes of the endothelium. Intactness of the endothelial structure was demonstrated by computer-aided reconstruction of a series of consecutive optical sections. These results indicate that analysis of unfixed brain capillaries by confocal microscopy offers the possibility of assigning the presence of membrane receptors to either the luminal or the basolateral plasma membrane domain.