The nature of the receptor sites for several agglutinins is characterized by hemagglutination inhibition assays. The inhibitory activity of human erythrocyte glycoproteins,from which sialic acid, sialic acid and galactose or alkali-labile oligosaccharides have been removed, is compared to the inhibitory effect of compounds with known structure. It is shown that the lectin from Arachis hypogea and anti-T bind to alkali-labile galactosyl-residues. Agglutinins from Bauhiniapurpurea and variegata (non- or N-speciflc),Maclura aurantiaca, Iberis amara, sempervirem, umbellata hybrida and umbellata nana(M- or nonspecific), Moluccella laevis (A- plus N-specific), Helix pomatia, Helix aspersa,Helix lucorum and Caucasotachea atrolabiata interact with alkali-labile V-acetylgalactosamine. The results obtained with the anti-A agglutinins from various snails suggest that human erythrocyte glycoproteins contain, besides the alkali-labile tetrasaccharide, a peptide-linked sialyl-N-acetyl-galactosaminyl-residue. The investigations do not allow a precise definition of the receptor sites for the lectins having M- or N-specificity.