Based on the fact that schistosomiasis patients in both the acute and chronic phase of the infection show a strong humoral immune response against the gut-associated circulating cathodic antigen, a simple and sensitive dot immunobinding assay for schistosomiasis japonica was developed. Circulating cathodic antigen that had been purified by immunoadsorption using monoclonal antibody was biotinylated with biotin aminocaproylhydrazide via the carbohydrate moiety of the antigen. Serum samples dotted onto nitrocellulose strips were then tested in an assay involving a combined incubation step of biotinylated antigen and streptavidin peroxidase, and a subsequent staining; the total assay time was 1.5 hr. Assaying the sera of 105 uninfected controls and 104 Schistosoma japonicum-infected individuals showed a specificity of 99.0%, and sensitivities of 96.2% (acute infections) and 94.1% (chronic infections). The described assay is economic, rapid, and reproducible and lends itself to use under field conditions.