Hydatidosis is a major public health problem with a worldwide distribution in humans. The purpose of this study is to investigate the circulating antigen in the sera of CE patients. This study was performed on 188 sera which were taken from 181 patients who attended the various departments of Ege University of Medical Faculty and to the laboratory of Parasitology department with the suspicion of Cystic Echinococcosis (CE). Sera were tested with IHA and ELISA for Circulating antibodies and with Circulating Antigen-ELISA (CAg–ELISA) test for circulating antigen. All the patients were questioned personally and clinical data were obtained from the departments where they operated to confirm the diagnosis of CE and 53 serum specimens were shown to be taken from CE proven cases. Hydatid Cyst Fluid (HCF) harvested from the fertile cysts in the liver of the infected sheep was used both in preparing the ELISA and IHA tests for detecting antibodies and for immunizing the rabbits to obtain immune serum. Specific polyvalent immunoglobulin’s required for CAg–ELISA test in detecting the circulating antigen for CE in sera, were obtained from the rabbits which were immunized against hydatid cyst antigen and used after being purified with ammonium sulphat (NH₄)₂ So₄ precipitation and gel filtration colon chromatography performed in the molecular biology department.  In 82 specimens with ELISA and in 81 specimens with IHA presence of antibodies, over the cut-off values, in varying titrations were shown out of 188 specimens. In 2(1.1%) patients who were found to negative by both IHA and ELISA, presence of CE was shown after the surgical operation. The specifity (79.26%) and sensitivity (88.68%) of IHA and specifity (78.52%) and sensitivity (90.57%) of ELISA tests were found. In 24(45.6%) specimen with CAg-ELISA test circulating antigen were detected in values over the cut-off level. All patients with positive CAg-ELISA are also confirmed with the personal data obtained from either personally or from the clinics where they were operated.  These results indicated that it will be great help to assess antigen detecting tests in additional to antibody detecting tests in the serodiagnosis of patients with suspicion of CE.