Surface antigens of three stages of three species of the filarial nematode genus Brugia have been analysed by radio-iodination and immunoprecipitation. These surface antigens have been shown to be characteristic for each stage by polyacrylamide gel electrophoresis. For example, infective larvae and adult worms have relatively complex patterns while microfilariae have few bands which are not found when other stages are radio-isotope labelled by the same technique. The surface antigens of Brugia malayi, B. timori and B. pahangi adult worms are all closely homologous, as are the surface antigens of infective larvae of the same three species, and of microfilariae of B. malayi and B. pahangi. Immunoprecipitation revealed that antibody raised in mice against one stage or species reacted with surface antigens from other stages and species. For example, sera raised against B. pahangi male adults reacted strongly with surface antigens from all three species. This cross-reactivity was dominant despite the apparent stage-specificity of the surface pattern seen on SDS-PAGE analysis. Moreover, in cross-immunization experiments, infective larvae were able to stimulate a secondary antibody response in mice previously primed with microfilarial surface antigens. The major microfilarial surface antigens (of mol. wt 65−70000 Daltons) were recognized by serum antibody from microfilariae-, infective larvae- or adult-infected animals. Thus, although the dominant antigens from each stage are of different molecular weight, cross-reactions with stage-specific antisera suggest that there must be shared epitopes on Brugia surface antigens from each stage. Such shared antigenic determinants dominate the immune response, although other evidence, including the differences in molecular weight, indicates the existence of stage-and species-specific components.