To investigate the distribution of C33 antigen of HCV in serum and liver tissues from normal subjects and patients with different diseases.Balb/C mice were immunized by HCV core antigen peptide C33(33 amino acids) which was conjugated with mice serum albzmin by a heterotypic difunctionalconnector(SPDP). Two strains of monoclonal antibody to C33 were produced. By using one of them as coated and the other as HRP(horseradish peroxidase)-labelled antibodies, an ELISA was developed to detect C33 antigen of HCV in 1236 serum samples from patients.The detectable rate of peptide C33 was 14.0%(6/43) in acute hepatitis C group, 8.8%(35/396) in anti-HCV-IgG positive group, 6.5%(4/62) in chronic hepatitis C group, 4.3%(1/23) in leukemia group, 3.3%(19/576) in hemodialysis group, 1.5%(2/136) in other diseases group, and 1. 4%(6/438) in blood donors. Results of immunohistochemistry and immunofluorescence for liver tissues obtained from biopsy showed that C33 peptide was found in nucli, cytoplasm and membranes of hepatic cells, scattered or laterally concentrated in the cytoplasm.The detection of C33 antigen of HCV may consider as one of the markers of HCV infection.