Abstract: A hybridoma cell line secreting an IgM monoclonal antibody designated NP30 was ob- tained from a fusion of SP2/O and spleen cells of a BALB/c mouse chronically infected with schistosoma japonicum for one and a half year and identified by screening with immunized rabbit sera against gut-associated antigen (GAA) and soluble egg antigen (SEA) of S. japonicum, indicating that the NP30 was an anti-anti-antigen or anti-antibody. NP30 was further determined to be an anti-idiotypic antibody (anti-id) which was serologically and functionally identical to GAA, so that it could be portraied as the internal image of GAA, which might have the potential to be used as an antigenic reagent in immunodiagnostic assays, of schistosomiasis japonica.