A murine hybridoma-derived antibody (IPH.134) has been produced which has apparent high binding specificity for an extract of Schistosoma japonicum adult worms. No binding was detected to extracts of S. japonicum eggs or to extracts from other adult trematodes (Fasciola hepatica, Paragonimus westermanii, Clonorchis sinensis, and Schistosoma mansoni) and several other helminths and protozoa. When sera from two series of S. japonicum-infected Philippine patients (19 and 20 patients, respectively) were tested for inhibition of binding of 125I-labeled IPH.134 to S. japonicum adult worm antigen, a low (10%) false-negative rate was obtained. In the 19-patient series, the 4 patients with highest fecal egg counts had high inhibitory activity in their sera. In the 20-patient series, the 3 patients with prominent disease had high inhibitory activity in their sera. Evidence was obtained that IgG anti-S. japonicum antibodies (rather than circulating antigens, immune complexes, or anti-idiotypic antibodies) were most likely to be responsible for serum inhibitory activity in the test. No false-positive reactions have been obtained with pooled or individual sera from patients infected with numerous parasites other than S. japonicum, although no information is yet available on the inhibitory activities of sera fro S. mansoni- or S. hematobium-infected individuals. On the basis of the data obtained to date, it is a reasonable prediction that the molecule or determinant to which this hybridoma antibody is directed will be a useful immunodiagnostic antigen for schistosomiasis japonica in the Philippines. A test based on detection of serum antibodies to this antigen should have high specificity and may provide additional information on the level of infection or disease status in patients.