A novel lectin from the mushroom Marasmius oreades (MOA) has been shown to bind to human blood group B oligosaccharides [1]. In the present work we examine the binding of a series of analogues of the blood group B-trisaccharide, alphaGal(1-3)[alphaFuc(1-2)]betaGal-OR (1, R = (CH2)8COOMe). MOA was biotinylated and immobilized on a micro column (9.8 microL) for evaluation by Frontal Affinity Chromatography-Mass Spectrometry (FAC-MS) [2]. The trisaccharide 1 was found to be the epitope needed for maximum recognition (Kd = 3.6 microM). A series of synthetic deoxygenated and O-methylated analogues of the B-trisaccharide (R = OMe) were then screened against the lectin, and the key structural elements for binding were determined. OH-4 of the beta-Gal residue and OH-2 of the alpha-Gal residue were found to be critical for recognition. The FAC-MS technique also proved powerful in evaluating mixtures of compounds. Since the solution NMR structure and crystal structure of the B-trisaccharide are known [3], we propose the specific surface of the trisaccharide that is recognized by the lectin.