SummaryA hybridoma‐derived monoclonal antibody, 1.134, with apparent specificity forSchistosoma japonicumadult worms has immunodiagnostic potential for schistosomiasis japonica in the Philippines. Sera from known infected individuals will inhibit the binding of125I‐labelled 1.134 to a crude adult worm extract (AWE) in a competitive radioimmunoassay (CRIA), although a 10% false negative rate has been noted. Another monoclonal antibody, P.41, which produces circumoval precipitation (COP) reactions withS. japonicumeggs was shown previously not to he useful in identification of heavily infected individuals using a CRIA with extracted egg antigens (EA). However, the125I‐P.41/EA assay has now been demonstrated to be capable of detecting a small subset of infected persons. Thus, four infected individuals with serum COP reactions shown to be consistently of the bleb typeonly(and with no segmented precipitates seen in the optimized COP test) have serum inhibitory activity in the P.41 CRIA. These same sera are negative in the I.134 CRIA.Unsuccessful attempts have been made to substitute a large Pool of affinity purified anti‐I.134 idiotypic (Id) antibodies for antigen (i.e. AWE) in the immunodiagnostic I.134 CRIA. However, anti‐Id CRIAs have been shown to be useful in monitoring the isolation of targetantigensof hybridoma anti‐schistosome antibodies. The availability of a range of hybridoma antibody‐based CRIAs will greatly Facilitate the quantitative analysis of anti‐S. japonicumantibody specificities in sera from clinically defined patient groups and the isolation of antigens of immunoparasitological interest.