Abstract: For the first time, the determination of circulating antigen in patients with cysticetocsis using specific monoclonal antibody against cysticercus antigen (CCy1) based on inhibitive ELISA was reported. The circulating antigens were detected in the sera from 83 patients with cysticercosis, the positive rate being 71.1%. The range of the detectable serum antigen concentrations was 0.16-128μg/ml. In 41 cases of cysticercosis patients, circulating antigens were determined in both sera and cerebrospinal fluids, the positive rates being 68.3% and 78% respectively, the total positive rate being 90.2%. (Fig.l) In 114 sera from normal persons, circulating antigen was absent. In 30 sera from patients with hydatidosis, 30 with clonor-chiasis, 20 with schistosomiasis japonica, 24 with paragonimiasis, and 5 sera collected from non-parasitosis patients, serum circulating antigen was not detectable. In another 10 non parasitosis patients, circulating antigen was not found in cerebrospinal fluid. After chemotherapy for one half to one year, in 21 out of 23 cases of cysticercosis patients, the circulating antigen levels in sera dropped to zero, except for two cases having an antigen evel of 0.64μg/ml and 1.6μg/ml, respectively. Our results showed that the determination of the circulating cysticercus antigen was very specific for the diagnosis of active infection of cysticercosis and could be used as a rational tool in monitoring the effectiveness of chemotherapy.