Glycopeptides and oligosaccharides of either the N-acetyllactosaminic or the oligomannosidic type derived from glycoproteins containing the N-glycosylamine linkage were used to define the specificity of different lectins (concanavalin A, Lens culinaris agglutinin, Vicia faba agglutinin, Pisum sativum agglutinin, Ricinus communis agglutinins, soybean agglutinin, wheat germ agglutinin, Solanum tuberosum agglutinin, Datura stramonium agglutinin, Lotus tetragonolobus agglutinin, Ulex europeus agglutinin) by studying the inhibition of human red blood cell agglutination by these structures. The results obtained show that lectins considered 'identical' in terms of monosaccharide specificity, possess the ability to recognize fine differences in more complex structures. In fact, different lectins are able to recognize different saccharidic sequences on the same glycan structure. As these sequences are likely to be common to numerous glycoproteins, including cell membrane glycoproteins, the results obtained with lectins in the study of cell surface carbohydrates have to be very carefully interpreted. Moreover, our results confirm previous data on the spatial configuration of the glycan moiety of glycoproteins deduced from the construction of molecular models: the fact that oligosaccharides bearing an alpha-NeuAc-(2 leads to 6)-Gal unit are more powerful inhibitors than oligosaccharides bearing an alpha-NeuAc-(2 leads to 3)-Gal unit could be related to the high rotational freedom of alpha-2,6 linkage; the observation that glycoasparagines, glycopeptides and glycoproteins possess a higher affinity for lectins than the related oligosaccharides could be explained by the fact that the glycan--amino acid linkage leads to structures more rigid than those of the oligosaccharides themselves.