Ultrastructural localization of immunoglobulin G (IgG) in the postcapillary venules (PVC) of normal and nude mouse lymph nodes was examined in thin sections which had been stained sequentially with primary exposure to biotin-conjugated goat anti-mouse IgG serum and a secondary immersion in a ferritin-linked avidin (IF). In the normal mouse PCV, reaction products showing IgG-binding sites were detected as large IF clusters on the luminal membrane of the endothelial cells, as small IF clusters in the intercellular spaces between the endothelial cells and some migrating lymphocytes, and as uniform precipitates of large numbers of IF particles in the basement membrane. By contrast, nude mouse PCV retained a few or almost no IgG-binding sites in portions comparable to those observed in the normal mouse. A relatively large number of plasma cells in the lymphoid stroma of both strains possessed large IF clusters on the cell surfaces. The significance of the IgG localization in the PCV of the two strains is discussed in relation to the mechanism of lymphocyte recirculation.