Lectins are proteins and glycoproteins of non-immune origin which bind specifically to carbohydrate residues, agglutinate cells and/or precipitate complex carbohydrates. Lectin-binding patterns in normal, hyperplastic and neoplastic endometria were studied using four biotinylated lectins (Con A, LCA, e-PHA, l-PHA) and the avidin-biotin-peroxidase technique. Canavalia ensiformis agglutinin (ConA) and Lens culinaris agglutinin (LCA) reacted strongly with the luminal borders and the cytoplasm of epithelial cells but, whilst in normal and benign endometrial tissues the cytoplasmic staining was confined to the apical and the basal aspect of the cells, in endometrial carcinomas and in some atypical hyperplasias lectin binding also occurred in the lateral cytoplasm (Con-A-lat), although in differing proportions of cells. Interestingly, extensive Con-A-lat in the tumour cells was much more frequent in non-endometrioid carcinomas (P<0.05) and was significantly associated with poor histological differentiation (P<0.0001), low oestrogen and progesterone receptor content (P<0.01 and P=0.0001, respectively) and an unfavourable long-term survival (P<0.05). With Phaseolus vulgaris erythroagglutinin (e-PHA) and leucoagglutinin (l-PHA) a linear, rather inconsistent, staining at the level of the basement membranes was observed in the glands: this, also noted with LCA, appeared intact in normal and hyperplastic glands without cytological atypia, and fragmented or absent in malignant glandular structures and in most hyperplastic glands showing cytological atypia. It is concluded that changes in the distribution of lectin-binding molecules in the endometrial cells are associated with the malignant state, whilst the extent of Con-A-lat reflects the biological behaviour of the tumours.