The binding of a hybridoma-derived antibody (designated IPH.134-18-6) to an extract of the adult worn of Schistosoma japonicum has been further characterized. This antibody has immunodiagnostic potential for detection of infection with S. japonicum in the Philippines since the binding of labeled hybridoma antibody to a crude adult worm extract in a solid-phase radioimmunoassay (RIA) is inhibited by sera from greater than 90% of known infected individuals, and no false positive reactions have yet been observed. Moreover, most patients with high fecal egg outputs or prominent disease have high serum inhibitory activity. All evidence indicates that IgG antibodies are the serum inhibitors in the competitive RIA. By estimating the amount of inhibitory activity (antibody?) of IPH.134-18-6 type in sera of infected patients and animals, the conclusion was reached that the antigen to which this hybridoma is directed is a strong immunogen. Moreover, no evidence of genetic unresponsiveness was obtained in strains of inbred mice infected with S. japonicum. IPH.134-18-6 does not bind to S. japonicum egg extracts. However, the actual stage and species specificity of this antibody has not yet been determined unequivocally and must await the screening of sera from individuals with monospecific S. mansoni and S. hematobium infections as well as assays using schistosomules, male and female adult worms and juveniles.