A range of excretory-secretory (ES) antigens have been characterised following in vitro culture of adult Brugia pahangi filarial nematodes in serum-free medium. Analysis by radioiodination, sodium dodecyl sulfate polyacrylamide gel electrophoresis, and immunoprecipitation of purified macromolecules with antibodies from human and experimental animal infections reveals both host and parasite components. Two host molecules appear by molecular weight and immunoprecipitation analysis to be immunoglobulin and serum albumin, presumed to be taken up from the jird host from which adult worms were recovered. A further prominent component, of 19 kDa, reacts with neither anti-host nor anti-filarial antibodies, and may represent a non-immunogenic parasite product. Three additional bands, although less intensely radiolabelled, did prove to be consistently antigenic, with apparent molecular weights of 15, 29 and 40 kDa. A further ES antigen, which does not take up radio-iodine or lend itself to electrophoretic analysis, has also been detected. This molecule reacts in a immunoradiometric assay in which monoclonal antibody directed against a repetitive epitope acts both to capture and indicate antigen presence. The same antibody, Bp-1, may also be employed to detect circulating antigen in the serum of animals experimentally infected with Brugia pahangi, and in the serum of patients with each of the three species of human lymphatic filariasis, Brugia malayi, Brugia timori and Wuchereria bancrofti.