The binding of 10 different lectins to the surface of microfilariae of Wuchereria bancrofti has been investigated. Wheat germ agglutinin (WGA) and Helix pomatia lectin (HPA) bound specifically to the sheathed microfilariae indicating the presence of N-acetyl-D-glucosamine and N-acetyl-D-galactosamine respectively on the surface. Exsheathed microfilariae did not react with any of the lectins. Treatment of sheathed microfilariae with proteases resulted in increased binding of WGA and HPA. Such treated microfilariae showed a weak binding of Concanavalin A (Con A), and lectins of lentil (LCH) and of Limulus polyphemus (LPA). Sheathed microfilariae incubated with sera of people living in endemic zones of filariasis but with no apparent evidence of infection (endemic normals), or with sera of chronic elephantiasis patients, or with their respective gamma globulin fractions, bound Con A and LCH. These lectins bound weakly to exsheathed microfilariae under the same conditions. Binding was due to the mannose components of the specific immunoglobulins of the sera which coated the microfilariae. However, microfilariae when incubated with sera or their globulin fractions from non-endemic normals (NEN), or from microfilarial carriers, did not bind Con A and LCH, suggesting that specific immunoglobulins were neither present in NEN sera nor in significant amounts in sera of microfilarial carriers.