The affinity of Bandeiraea (Griffonia) simplicifolia lectin-I isolectin B4 (BSI-B4) for the isomer of human blood group B active disaccharide (B, Gal alpha 1-->3Gal), the Gal alpha 1-->4Gal galabiose ligand, was studied by quantitative precipitin (QPA) and precipitin-inhibition assays. When human blood group B, P1 and H active glycoproteins were tested by OPA. BSI-B4 reacted strongly with both the B active glycoprotein purified from human ovarian cyst fluid and a P1 active glycoprotein isolated from sheep hydatid fluid and precipitated over 86% of the lectin nitrogen added. The P1 active glycoprotein-BSI-B4 interaction was inhibited by both Gal alpha 1-->3Gal alpha 1-->methyl and Gal alpha 1-->4Gal disaccharide indicating that BSI-B4 is not only reacting with Gal alpha 1-->3Gal disaccharide, but also recognizing Gal alpha 1-->4Gal. The galabiose sequence is frequently found in the carbohydrate chains of many glycosphingolipids located at the mammalian cell membranes such as intestinal and red blood cell membranes, for E. coli ligand binding and toxin attachment.