The extent of structural and immunological similarity between surface antigens of three species of Brugia filarial parasites was studied by lodogen-mediated surface labelling of adult worms of B. malayi, B. pahangi and B. timori. The close homology and cross-reactivity between these antigens reported in previous surface labelling studies with Bolton-Hunter reagent, was verified in this system. The surface antigens of adult B. pahangi are also recognised by antibody from patients with Wuchereria bancrofti, the major human lymphatic filariae, and by antibody to Loa loa and Mansonella perstans. Further experiments have begun to establish the boundaries of these cross-reactions: antibodies to nonfilarial nematodes such as Trichinella, Necator and Strongyloides does not recognise the adult surface antigens; however, although most anti-Onchocerca sera show little or no reaction to the major (29 kDa) surface antigen, there is consistent reactivity to the secondary 20 kDa antigen, and extensive recognition of a minor antigen of 15 kDa.