Background Hepatitis C virus (HCV) core antigen assays have been produced to exclude infectious donations collected during the preseroconversion window phase (PWP). For the same purpose, we evaluated the specificity and sensitivity of a novel hepatitis C virus NS3 antigen detection immunoassay and the application of this assay in clinical diagnosis.Methods Samples from 77 healthy subjects, 173 anti-HCV positive patients and 3708 hepatitis patients other than HCV positive were tested with the HCV NS3 antigen assay. Some HCV NS3 antigen positive samples were further validated with HCV-RNA, neutralization and immunodot assays. Twenty-five sequential samples from 11 HCV NS3 antigen positive patients were subjected to kinetic study.Results Only 48 (1.3%) of 3708 anti-HCV negative samples were positive for HCV NS3 antigen. Among them, 44 of 3030 samples from patients only infected with HBV were HCV NS3 antigen positive, 4 of the 445 samples from patients infected with other type hepatitis were HCV NS3 antigen positive. In addition, 42 (24.3%) of 173 anti-HCV positive samples were HCV NS3 antigen positive and all 77 samples from healthy subjects were negative to HCV NS3 antigen assay. Of the 15 HCV NS3 antigen positive samples, 9 (60%) were HCV-RNA positive. The neutralization and positive percentage of immunodot assay for 23 HCV NS3 antigen positive sera were 87.0% (20/23) and 69.6% (16/23)respectively. Of the 25 sequential samples from 11 HCV NS3 antigen positive patients, there was a negative correlation between the OD values and the duration of test (r=-0.989, P＜0.05), and there were correlations among their HCV NS3 antigen, HCV-RNA and anti-HCV titres. The anti-HCV antibodies of two sera were detected while their OD values of HCV NS3 antigen decreased gradually.Conclusions The HCV NS3 antigen detection assay showed perfect specificity and high sensitivity. Thus, it would be useful and economical as a routine test in laboratories for early diagnosis of HCV infection and prevention.