The existence of terminal D-mannosyl 6-phosphate groups (Man-6-P) was required (for an inhibitor) to exert a strong inhibitory potency against the binding of bovine serum albumin (BSA) conjugated with 17 molecules of penta-D-mannose 6-phosphate [(M5P)17-BSA] to the Man-6-P receptor in rabbit alveolar macrophages. In addition, the underlying oligosaccharide structures, such as linkage mode between the nonreducing sugar group and the penultimate sugar residue, and the length of sugar chain also affected the inhibitory potency in this system. In general, the oligosaccharides with an alpha-(1----2)-linked Man-6-P unit gave stronger inhibitory potencies than those with an alpha-(1----3)- or alpha-(1----6)-linked Man-6-P unit. Trisaccharides containing a terminal Man-6-P group were more potent inhibitors than disaccharides. A synthetic, branched, and divalent ligand, which does not have a penultimate sugar residue, gave about the same level of inhibitory potency as Man-6-P itself. The "cluster effect" was observed in this system, i.e., as the number of Man-6-P units conjugated to BSA [(Man-6-P)5,5,8, and 46-BSA] increased, the stronger inhibitory potencies were observed with decreasing I50 values of 1.93, 1.36, and 0.0345 microM, respectively. Synthetic divalent oligosaccharides also showed higher inhibitory potencies than the corresponding monovalent oligosaccharides.