The binding properties of a glycoprotein with blood group P 1 specificity isolated from sheep hydatid cyst fluid with Gal and GalNAc specific lectins was investigated by quantitative precipitin and precipitin inhibition assays. The glycoprotein completely precipitatedRicinus communis agglutinin (RCA 1 ), Abrus precatorius agglutinin (APA) and Mistletoe toxic lectin-I (ML-I). Only 1.0 [mu ]g of P 1 glycoprotein was required to precipitate 50% of 5.1 [mu ]g ML-I nitrogen. It also reacted well with abrin-a and ricin, precipitating over 73% of the lectin nitrogen added, but poorly or weakly with Dolichos biflorus (DBL), Vicia villosa (VVL, a mixture of A 4 , A 2 B 2 and B 4 ), VVL-B 4 , Arachis hypogaea (PNA), Maclura pomifera (MPL), Bauhinia purpurea alba (BPL) and Wistaria floribunda (WFL) lectins. When an inhibition assay in the range of 5.1 [mu ]gN to 5.9 [mu ]gN of lectins (ML-I, abrin-a; ricin, RCA 1 , and APA, and 10 [mu ]g P 1 active glycoprotein interaction was performed; from 76 to 100% of the precipitations were inhibited by 0.44 and 0.52 [mu ]mol of Gal [alpha ] 1 [rarr ] 4Gal and Gal[beta ]1 [rarr ] 4GlcNAc, respectively, but not or insignificantly with 1.72 [mu ]mol of GlcNAc. The Gal [alpha ] 1 [rarr ] 4Gal disaccharide found in this P 1 active glycoprotein is a frequently occurring sequence of many glycosphingolipids located at the surface of mammalian cell membranes, especially human erythrocytes and intestinal cells for ligand binding and microbial toxin attachment. The present finding suggests that the Gal [alpha ] 1 [rarr ] 4Gal[beta ]1 [rarr ] 4GlcNAc sequence in this P 1 active glycoprotein is one of the best glycoprotein receptors for three toxic lectins (ricin, abrin-a, and ML-I) as well as for APA, and RCA 1 , and the result of inhibition assay implies that these lectins are recognizing part or all of the Gal [alpha ] 1 [rarr ] 4Gal[beta ]1 [rarr ] 4GlcNAc sequence in the P 1 active glycoprotein.