PV-1 is a novel endothelial protein shown by immunocytochemical tests to be specifically associated with the stomatal diaphragms of caveolae in lung endothelium. Although the highest expression levels of both mRNA and protein are in the lung, PV-1 also has been found to be expressed in other organs. Using a specific antibody to the extracellular domain of PV-1, we have extended the survey on the presence of this protein at light and electron microscope level in several rat organs. Here we show that by immunofluorescence the antibody recognizes with high specificity the endothelium of the fenestrated peritubular capillaries of the kidney and those of the intestinal villi, pancreas, and adrenals. By immunolocalization at electron microscope level, the antibody recognizes specifically the diaphragms of the fenestrae and the stomatal diaphragms of caveolae and transendothelial channels in the endothelia of these vascular beds. No signal was detected in the continuous endothelium of the heart, skeletal muscle, intestinal muscularis, or brain capillaries or the nondiaphragmed fenestrated endothelium of kidney glomeruli. Taken together, our findings define the only antigen to be localized thus far in fenestral diaphragms. They also show that the stomatal diaphragms of caveolae and transendothelial channels and the fenestral diaphragms might be biochemically related, in addition to being morphologically similar structures.