Monoclonal antibody 128C3/3/21 in an antigen-capture ELISA was used to detect circulating antigen in individuals infected with Schistosoma mansoni. This antibody recognizes a carbohydrate epitope expressed on the major group of acidic egg glycoproteins and on glycoproteins and glycolipids in all other stages of parasite development. The overall sensitivity of the assay was 78%, with a sensitivity of 100% for patients excreting >100 egg/g feces (EGF) and 72% for those excreting <100 EGF. By increasing the degree of antibody biotinylation, the authors have now achieved sensitivities of 92.4% overall and 82% for those excreting <100 EGF. A direct increase in the mean level of circulating antigen was found with increasing egg counts. The difference between those excreting >100 EGF (53 individuals) and those excreting <100 EGF (39 cases) was statistically significant (P<0.01). None of the control sera (23 uninfected individuals and 16 patients infected with other parasites) had circulating antigen levels >80 ng/ml. Thus, the test specificity was >99%. The test accuracy was 94.7%, the positive predictive value 100%, and the negative predictive value 84.8%.