A comparison was made between properties of a recently discovered Entamoeba histolytica lectin which has a carbohydrate specificity for N-acetylglucosamine oligosaccharides and the previously found toxin-like principle of the ameba. A separation between these two activities was achieved upon subcellular fractionation by high speed centrifugation of freeze-thawed disrupted E. histolytica trophozoites (strain HM-1). Practically all of this lectin activity, as determined by hemagglutination of glutaraldehyde-fixed human erythrocytes, was found associated with the sedimented membrane fraction. This fraction did not affect monolayers of tissue-cultured mammalian cells. On the other hand, the soluble supernatant solution caused extensive damage to the tissue-cultured cells (change in morphology and detachment of cells). Both the lectin and toxin activities were heat-labile and their activities were preserved by the presence of reducing agents and proteolytic enzyme inhibitors. In contrast to the toxin, the isolated lectin was inactive at pH 7.2 and active only at pH 5.7–6.0. Both the lectin and toxin were inhibited by a number of macromolecular compounds such as chitin, peptidoglycan, bovine serum and an IgA fraction isolated from human colostrum. Only the lectin activity, however, was inhibited by low molecular weight chitin oligosaccharides (GlcNAc)n=2–6 or by lysozyme-digested peptidoglycan subunits. Moreover, fetuin and a ganglioside mixture extracted from ox brain were found to inhibit only the toxin-like activity. The IgG fraction of sera from patients with invasive amebiasis neutralized both lectin and toxin-like activities, while IgG from normal sera failed to neutralize either activity. Although our results indicate that in E. histolytica, lectin and toxin are two separate activities, both of them share a considerable number of properties which does not exclude the possibility that they may be related.