HCV core antigen (protein) assay using monoclonal Ab to HCV core region was developed. The detection ratio of HCV core protein was 209 (81%) [4 (80%) in AH, 85 (82%) in CH,64 (88%) in LC, 56 (74%)) in HCC]. HCV core protein dose in HCC (231±337 RFI) was less than CH (380±521) and LC (318±429) (p<0.05). The detection limit of this assay was between 10 3 -10 4 copies/ml HCV RNA and more sensitive than bDNA assay. HCV core protein dose correlated with the quantitation of serum HCV RNA levels (both bDNA and RT-PCR, p<0.01). HCV core protein and HCV RNA showed parallel change in IFN-treated patients and serum ALT level followed to HCV core protein. This HCV core protein assay is not only sensitive and specific but simple, quick (assay time 3hr) and cost-effective. It is very useful for the estimation of HCV viremia.