The carbohydrate moieties of microfilariae (Mf) and infective larvae (L3) have been investigated by lectin-binding technique. Mf derived from three sources, namely, uteri (in utero), released in vitro from adults and from blood of rodents infected with Litomosoides carinii were examined by using fluoresceinated lectins. Wheat germ agglutinin (WGA) bound to these Mf and the binding was inhibited by N-acetyl glucosamine. In addition to WGA, Concanavalin A (Con A) and lentil lectin (LCH) bound to in vitro-released and in utero-derived Mf showing the presence of mannose moieties on their surface. In utero-derived Mf also showed binding with the agglutinins of Limulus polyphemus (LPA), peanut (PNA), Ricinus communis (RCA), Helix pomatia (HPA), Soyabean (SBA) and Dolichos biflorus (DBA) but not to that of Ulex europaeus (UEA) indicating the presence of additional carbohydrate molecules like sialic acid, galactose and N-acetyl galactosamine on their sheath. None of the lectins bound to the cuticle of exsheathed Mf. Treatment of blood-derived and in vitro-released Mf with certain proteases exposed additional binding sites for SBA, HPA, Con A and LCH. In case of L3, only PNA bound to the larvae isolated from infective mites Bdellonyssus bacoti, and the binding was inhibited by D-galactose. No such binding of the lectins was seen to the larvae that migrated to the pleural cavity of jirds indicating that there is considerable change on the parasite surface during their migration in the vertebrate host. Sheathed Mf and mite-derived L3 when incubated with immune rat sera, bind Con A and LCH lectins possibly due to the mannose components of the specific immunoglobulins that coat onto the Mf and L3.