Immunohistochemistry of spleen using the anti-Duffy MoAb, anti-Fy6. Analysis was done on both freshly obtained and archival specimens of spleen. Anti-Fy6 reacts with specialized endothelial cells (littoral cells) lining the sinusoids in the red pulp. Endothelial cells lining arterioles, venules, arteries, and veins did not stain with anti-Fy6. Similar staining was observed with endothelial cells of postcapillary venules in every organ examined thus far. Immunohistochemical staining of endothelial cells in tissue obtained from Duffy-negative individuals (by RBC typing) was identical to that observed in tissue obtained from Duffy-positive individuals. In contrast to the sinusoids of the spleen, endothelial cells lining hepatic sinusoids did not stain.