Abstract: Circulaing antigen in sera from acute, chronic and late stages of schistosomiasis patients was detected by direct dot-ELISA with monoclonal antibody 3D8A against schisto-some gut-associated cathodic antigen linked with peroxidase, the positive rates being 90.6 %, 83.2% and 30.7%, respectively. No positive reactions were found with sera from patients of clonorchiasis, malaria and non-parasitic diseases. The positive rate and the circulating antigen level in EPG100 group of patients were found to be higher than those in EPG 100 group. Circulating antigen became negative one year after praziquantel treatment in 84.0% of patients who showed negative fecal examination, while the other patients remained positive with decreasing tilers. The results indicated that the circulating antigen in sera from schistosomiasis patients of various stages can be detected by dot-ELISA with monoclonal antibody 3D8A against circulating schistosome gut-associated cathodic antigen. The authors concluded that the circulating antigen level was correlated with the intensity of infection and the efficacy of treatment.