The heparin-binding lectin, Anadarin MS, from the plasma of the marine clam Anadara granosa is purified through affinity chromatography on heparin-Sepharose 4B followed by gel filtration on a Sepharose 6B column. The purified lectin is a pentameric protein of native M(r) 300 kDa and is composed of identical subunits of 60 kDa. The pI value of this Ca(2+)-dependent lectin is 6.2. Anadarin MS agglutinates normal rabbit erythrocytes but not that of human. Aspartic acid, glutamic acid, histidine and glycine are the predominant amino acids. Unlike other reported heparin-binding lectins, Anadarin MS exhibits a unique and strict specificity for iduronic acid containing glycosaminoglycans. This lectin agglutinates infective promastigotes of Leishmania donovani exclusively and can therefore be used as a novel biochemical surface marker for this parasite.