Abstract: The 170 kDa adhesin subunit of the Entamoeba histolytica Gal/GalNAc adherence lectin is encoded by members of a gene family that includes hgl1, hgl2 and a newly discovered gene, hgl3. The DNA and encoded protein sequences of the hgl genes are disclosed. A number of proteins and peptide fragments of the adhesin as well as other functional derivatives, preferably produced by recombinant methods in prokaryotic cells are disclosed. A preferred peptide for a vaccine composition corresponds to amino acids 896-998 of the mature 170 kDa lectin and contains the galactose- and N-acetylgalactosamine-binding activity of the native lectin. These compositions are useful as immunogenic vaccine components and as diagnostic reagents. Methods are provided for a vaccine comprising one or more peptides of the lectin to immunize subjects at risk for infection by E. histolytica. Additionally, immunoassay methods are disclosed for measuring antibodies specific for an epitope of the lectin. These methods detect E.