The binding properties of Caragana arborescens agglutinin (CAA, pea tree agglutinin) were studied by enzyme linked lectinosorbent assay (ELLSA) and by inhibition of CAA-glycan interaction. Among glycoproteins (gps) tested, CAA reacted strongly with asialo bird nest gp, asialo rat sublingual gp, human Tamm-Horsfall Sd(a +) urinary gp (THGP) and asialo THGP that are rich in GalNAcα1→, GalNAcβ1→ and/or Galβ1→4GlcNAc residues. CAA also bound tightly with multi-valent Galβ1→4GlcNAc (mII) containing glycoproteins (human blood group precursor gps, asialo fetuin) and asialo ovine salivary glycoprotein (Tn, GalNAcα1→Ser/Thr), but CAA reacted poorly or not at all with sialylated glycoproteins tested. Of the sugars tested for inhibition of binding, Forssman pentasaccharide (F p, GalNAcα1→3GalNAcβ1→3Galα1→4Galβ1→4Glc) was the best. It was about 2.3, 9.5 and 52.6 times more active than Galβ1→4GlcNAc, GalNAc and Gal, respectively, and about 1.9 times more active than tri-antennary Galβ1→4GlcNAc (Tri-II). These results suggest that this agglutinin is mainly specific for F p, mII and Tn clusters. This property can be used to detect human abnormal glycotopes related to F p and unmasked mII/Tn clusters and to study cell growth and differentiation given the lack of toxicity of this lectin toward mouse fibroblast cells.