Antibodies directed towards the sheaths of microfilariae have been implicated in the elimination of circulating microfilariae, both in experimental and human filariasis. In the present study antisheath antibodies have been detected in human sera by indirect immunoperoxidase assay (IPA) using fixed Wuchereria bancrofti microfilariae as antigen. One hundred and eighteen sera collected from an area endemic for Bancroftian filariasis were tested. While 80% of sera from microfilariae carriers had no demonstrable antisheath antibodies, more than 80% of amicrofilaraemic samples (chronic filariasis cases and endemic normals) had antisheath antibodies. The antibody activity was found in IgG, IgM and also IgE isotypes. IgG subclass typing with monospecific antisera revealed significantly higher antisheath activity in IgG2 in comparison with other IgG subclasses. The determinants on sheathed microfilariae reacting with antisheath antibodies were found to be thermostable (100 degrees C for 30 minutes), resistant to protease treatment and significantly sensitive to sodium periodate treatment, indicating the possible role of carbohydrate moieties in eliciting protective antisheath antibodies in Bancroftian filariasis.