We have developed an immunohistochemical method for the in vivo and in vitro detection of glycosyl receptors in rat spleen and lymph nodes by using neoglycoproteins. The receptor in both organs recognized mannose coupled to bovine serum albumin (mannose-BSA), fucose-BSA, N-acetylglucosamine-BSA and to a lesser extent glucose-BSA, but not galactose-BSA or N-acetyl-galactosamine-BSA. In vitro neoglycoprotein-receptor binding was Ca2+ dependent and could be inhibited by mannan but not by mannose. Simultaneous staining with the monoclonal antibodies ED1, ED2 or ED3 revealed that only ED1- and ED3-positive macrophages were involved in the binding of neoglycoproteins. In the spleen, the marginal-zone macrophages and a subpopulation of the marginal metallophils possess glycosyl-binding receptors. In the lymph nodes, the medullary sinus macrophages and a subpopulation of the outer-cortex macrophages are able to bind neoglycoproteins.