The interaction of a series of synthetic, branched trisaccharides with five D-mannose-specific lectins was studied by precipitation-inhibition assay. The branched methyl alpha-D-mannotrioside, alpha-D-Manp-(1----3)-[alpha-D-Manp-(1----6)]-alpha-D-Man pOMe, the best inhibitor of the Con A-Dextran interaction, was 42 times more potent than alpha-D-ManpOMe, and 3-6 times more potent than the two trisaccharides substituted with D-glucosyl groups, and 8-15 times those with D-galactosyl groups. Surprisingly, methyl O-alpha-D-mannopyranosyl-(1----3)-alpha-D-mannopyranoside was bound to Con A 8-fold more avidly than methyl alpha-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 examined. The trisacchrides containing terminal, nonreducing, (1----3)-linked alpha-D-mannopyranosyl groups, i.e., alpha-D-Manp-(1----3)-[alpha-D-Glep-(1----6)]alpha-D-Manp OMe, alpha-D-Manp-(1----3)]-alpha-D-Galp-(1----6)]-alpha-D-ManpOMe++ +, and alpha-D-Manp-(1----3)-[alpha-D-Manp-(1----6)]-alpha-D-Man pOMe, were the best inhibitors of the snowdrop lectin (GNA)-D-mannan precipitation system. On the other hand, all branched trisaccharides exhibited very similar inhibitory potencies toward the daffodil lectin (NPA)-D-mannan interaction, whereas alpha-D-Manp-(1----3)-[alpha-D-Galp-(1----6)]-alpha-D-ManpOMe++ + and alpha-D-Manp-(1----3)-[alpha-D-Manp-(1----6)]-alpha-D-Man pOMe were somewhat better inhibitors than the other branched trisaccharides of the amaryllis lectin (HHA)-D-mannan precipitation reaction. (ABSTRACT TRUNCATED AT 250 WORDS)