Biotinyl derivatives of several lectins and avidin-horseradish peroxidase were used to study the localization of glycoconjugates in amyloid plaques and in neuritic tangles in brains of patients with Alzheimer's disease (AD), Downs syndrome (DS) and Gerstmann-Sträussler syndrome (GSS). The lectins tested recognize the following residues: beta-D-galactosyl [Ricinus communis agglutinin 120, (RCA-1) and peanut agglutinin, (PNA)]; alpha-D-galactosyl [Griffonia simplicifolia agglutinin (GSA)]; alpha-D-mannosyl greater than alpha-D-glucosyl [concanavalin A (Con A) and Lens culinaris agglutinin (LcH)]; N-acetyl- and N-glycolyl-neuraminic acid [Limax flavus agglutinin (LFA) and Limulus polyphemus agglutinin (LPA)]; N-acetyl-glucosaminyl and sialyl [wheat germ agglutinin (WGA)]; N-acetyl-D-galactosaminyl [Helix pomatia agglutinin (HPA) and Dolichos biflorus agglutinin (DBA)] and alpha-L-fucosyl [Ulex europeus agglutinin (UEA-1)]. The majority of lectins listed above bind preferentially to the peripheral area of AD plaques, whereas in plaques of DS they are mainly bound to central amyloid core. In neurofibrillary tangles of AD brains only residues recognized by WGA and HPA or DBA were found, whereas in DS brains, in addition to above mentioned, beta-D-galactose (RCA-1) and sialic acid (LFA) were also present. In brain microblood vessels the strongest reaction in endothelia appeared with UEA-1 and RCA-1, indicating the abundance of alpha-L-fucosyl and beta-D-galactosyl residues. In AD brains deposits of amyloid were noted in the wall of some blood vessels, where monosaccharide residues recognized by RCA-1, GSA, UEA and WGA but not by Con A and LFA were present. However, our studies of some organs (liver, kidney, heart and testes) of patients with generalized amyloidosis revealed a lack of these sugar residues. It indicates, that the composition of amyloid present in brains of AD is different to that in other organs in generalized amyloidosis.