The effectiveness of praziquantel in treating schistosomiasis is most commonly assessed by quantitating egg production or anti-schistosome antibodies in serum. We have used a monoclonal antibody (MAb)-based antigen-capture enzyme-linked immunosorbent assay (ELISA) for the serologic diagnosis of schistosomiasis, and to monitor the efficacy of praziquantel therapy in 49 individuals with parasitologically proven schistosomiasis. The MAb used, 128C3/3/21, recognizes a repeating carbohydrate epitope expressed at all stages of parasite development, and antibodies recognizing this epitope are found in the serum of infected humans. The overall sensitivity of the ELISA was 78%, with a sensitivity of 100% for patients excreting greater than 100 eggs/g of feces and 72% for those excreting less than 100 eggs/g of feces. The positivity of the ELISA was directly related to the fecal egg counts obtained on days -3, -2, and -1 before treatment with praziquantel, but there was no correlation between antigen levels and the clinical stage of the disease. After praziquantel treatment, we observed a highly significant correlation (P less than 0.0001) between the time elapsed since treatment and the decrease in antigenemia. Furthermore, although no eggs were detected in any of the stool specimens at week 12 after treatment, the antigen was detected in 21% of the treated patients (seven of 33 ELISA-positive patients). Antigen levels decreased over the 12-week period in six of these patients, whereas the antigen level increased with time in one individual. The persistence of antigenemia suggests that these individuals are either still clearing antigen or remain infected.(ABSTRACT TRUNCATED AT 250 WORDS)