SummaryAn IgG2amouse hybridoma‐derived antibody (designated I.134) has been identified which binds to Schistosoma japonicum adult worms and which has immunodiagnostic potential (for detection of antibody) in schistosomiasis japonica in the Philippines. The target epitope of this hybridoma antibody is contained in a 23 000 molecular weight protein of adult worms as analysed by one‐dimensional sodium dodecyl sulphate‐polyacrylamide gel electrophoresis of immunoprecipitates and a gel overlay technique. This adult worm antigen has been labelled biosynthetically using35S‐methionine as well as exogenously using lactoperoxidase‐catalysed radioiodination and the Bolton and Hunter reagent with intact worms. As anticipated, the low molecular weight protein antigen (designated Sj23) appears to be one of several major immunogenic proteins of worms which induce antibodies in infected Philippine