Labelled lectins were used for the histochemical distinction of glycolipids with various terminal hexoses in different storage diseases as well as for the demonstration of differences in the composition of cell surface and intercellular glycoproteins in tissues at various stages of differentiation, in malignancy and in functional subsets of cells. Thereby the detection of some glycoproteins (i.e. in primary sensory neurones, in parietal cells or in polymorphonuclear leukocytes), which were invisible by formerly used methods, was also possible. In malignant tissues lectin binding was either altered or the intracellular localization of the lectin binding sites was changed. These differences did not represent tumour specific markers, rather they were found to reflect glycoprotein structures characteristic for an activated functional state or for an earlier grade of differentiation. Some of such glycoproteins were isolated by lectin affinity chromatography and their structures were further analysed by means of biochemical methods. In malignancy a reduced molecular weight and changes in the monosaccharide composition of surface or intracellular glycoproteins were found to be the main differences. Antibodies produced against the lectin affinity isolated glycoproteins allowed the demonstration of structural relationship of glycoproteins with identical or diverse lectin binding pattern. They proved to be also useful for diagnostic purposes, especially for the differential diagnosis of anaplastic tumours.