Immunoelectron microscopic localization of gp-Fy in ultrathin cryosections of human lung. Antibody 6615 was used. (a) Extensive labeling of both apical and basal membrane domains of vascular endothelial cells (E) was seen. Membranes of red blood cells (RBC) were extensively labeled. (b) The endothelial cells (E) of a peribronchiolar venule exhibited strong labeling in both apical and basal membrane domains (arrows). An interstitial cell was unlabeled (I). BM, basement membrane. (c and d) When using preimmune serum, no labeling was seen in red blood cells (c) and endothelial cells (d). (Original magnification × 60,000.)