A lectin (carbohydrate binding protein) has been found in extracts of a number of axenically grown trophozoites of Entamoeba histolytica strains. The strains grown in Diamond's TYI-S-33 media were HK-9, 200:NIH and HM-1: IMSS. Strain HU-1: MUSC (HSC) was grown monoxenically in the same medium. The amoeba lectin agglutinates glutaraldehyde-fixed red blood cells. This activity is pH dependent, heat and oxidation sensitive and is destroyed by proteolysis upon auto-incubation. The relative agglutinating potency of the different strains was investigated. Strain HSC had the highest specific activity (210 units/mg protein) and strain HM-1 the lowest (14 units/mg). One unit of haemagglutinating activity is defined as the amount of leetin present in 1 ml of extract which will agglutinate 1 ml of 4 per cent red blood cells. Upon subcellular fractionation of the lectin present in extracts of strain HK-9, two thirds of the activity were detected in the soluble, non-sedimentable (100,000 x g, 60 min) fraction. Partial hydrolysate of chitin was found to inhibit the haemagglutinating activity. Among the oligosaccharides of N-acetylglucosamine, the trimer and tetramer were the most potent inhibitors. The lectin was purified approximately 300 fold by a one step affinity chromatography on a chitin column. The loading and elution from the column were based on the pH dependence of the lectin activity.