The adherence of Entamoeba histolytica to colonic mucins and to host cells appears to be predominantly mediated by a 170-kDa surface lectin of the amoebae. By using an antiserum to the purified lectin, the corresponding cDNA was isolated from an expression library of the pathogenic E. histolytica isolate HM-1:IMSS. Northern blot analysis indicated a transcript of approximately 4 kilobases, and Southern blot analyses suggested that multiple genes may encode the lectin or closely related proteins in HM-1:IMSS trophozoites. The cDNA-deduced amino acid sequence revealed an N-terminal signal peptide and a mature protein of 1270 amino acids corresponding to a molecular mass of 143 kDa, which comprises a short C-terminal cytoplasmic domain with potential phosphorylation sites, a transmembrane region, and a large extracellular portion with nine potential asparagine-linked glycosylation sites. The extracellular portion may be separated into a cysteine-poor domain and a cysteine-rich domain, the latter of which shows in part repetitive structural elements with a low degree of sequence homology to wheat germ agglutinin and to pDd63, a developmentally expressed protein of Dictyostelium discoideum.