The interaction of a series of synthetic, branched trisaccharides with 5 D-mannose-specific lectins was studied by pptn.-inhibition assay. The branched Me a-D-mannotrioside, a-D-Manp-(1->3)-[a-D-Manp-(1->6)]-a-D-ManpOMe, the best inhibitor of the Con A-Dextran interaction, was 42 times more potent than a-D-ManpOMe, and 3-6 times more potent than the 2 trisaccharides substituted with D-glucosyl groups, and 8-15 times those with D-galactosyl groups. Surprisingly, methyl-O-a-D-mannopyranosyl-(1->3)-a-D-mannopyranoside was bound to Con A 8-fold more avidly than Me a-D-mannopyranoside. However, the related pea lectin (PSA) was singularly different from Con A in its carbohydrate-binding activity, showing no significantly enhanced binding to any of the sugars examd. The trisaccharides contg. terminal, nonreducing, (1->3)-linked a-D-mannopyranosyl groups, i.e., a-D-Manp-(1->3)-[a-D-Glcp-(1->6)-a-D-ManpOMe, a-D-Manp-(1->3)]-a-D-Galp-(1->6)]-a-D-ManpOMe, and a-D-Manp-(1->3)-[a-D-Manp-(1->6)]-a-D-ManpOMe, were the best inhibitors of the snowdrop lectin (GNA)-D-mannan pptn. system. On the other hand, all branched trisaccharides exhibited very similar inhibitory potencies toward the daffodil lectin (NPA)-D-mannan interaction, whereas a-D-Manp-(1->3)-[a-Galp-(1->6)]-a-D-ManpOMe and a-D-Manp-(1->3)-[a-D-Manp-(1->6)]-a-D-ManpOme were somewhat better inhibitors than the other branched trisaccharides of the amaryllis lectin (HHA)-D-mannan pptn. reaction. Of the oligosaccharides studied, the linear trisaccharide a-D-Manp-(1->6(-a-D-Manp-(1->6)-D-Man appears to be the most complementary to the combining site(s) of NPA and HHA. [on SciFinder (R)]