SummaryA set of cross‐reactive antigens is described which are present in somatic extracts and in‐vitro secretions of the filarial nematodesBrugia pahangiandB. malayi.A monoclonal antibody reactive with a repeating epitope on these molecules readily detects circulating antigen in the serum of animals infected with lymphatic filariae, using either an immunoradiometric assay or enzyme‐linked immunosorbent assay (ELISA). This epitope has the immunological reactivity and chemical characteristics of the phosphorylcholine (PC) hapten. The anti‐PC monoclonal has been used to define the antigens bearing this epitope, and in chromatographic studies on material from extracts ofBrugiaadult worms, a heterogeneous profile of PC‐positive molecules are found. In sera fromBrugia‐infected jirds, an antigen with a native molecular weight of approximately 500000 is observed, which displays limited sensitivity to protease degradation. However, denatured samples on Western blots show a major parasite circulating antigen of Mr90000. The detection of this antigen in the presence of excess host antibody is also demonstrated, taking advantage of the stability of the target epitope to a range of treatments designed to dissociate and eliminate immun