A simple immunoassay method is provided which distinguishes pathogenic from nonpathogenic forms Entamoeba histolytica in biological samples. This assay utilizes monoclonal antibody preparations which are specific for designated epitopes of the 170 kd subunit of the Gal/GalNAc lectin. When pathogenic forms, specifically, are to be detected, at least one antibody which is immunospecific for an epitope unique to the forms of the 170 kd lectin found in pathogenic strains is used in the assay. The invention further includes monoclonal antibodies which are immunospecific for epitopes 1-3 of the 170 kd subunit of the Gal/GalNAc lectin of either pathogenic or nonpathogenic forms and to monoclonal antibodies specifically immunoreactive with epitopes unique to nonpathogenic derived 170 kd subunit, as well as purified forms of the Gal/GalNAc lectin from both pathogenic and nonpathogenic forms.