The ability of the total hepatitis C virus (HCV) core antigen assay was evaluated for monitoring the therapeutic responses of HCV-infected patients treated with interferon. The ability to detect and quantitate an independent structural protein component of HCV, in the presence of circulating antibodies, makes this assay a valuable new tool in diagnosis and treatment monitoring. Measurement of total core antigen showed a strong dynamic correlation with HCV RNA data and may serve as an alternative direct marker of viral infection. In addition, with the advent of additional treatment protocols, a rapid, reliable assay for changes in HCV load may permit more frequent patient assessment and tailoring of the therapeutic regimen. J. Med. Virol. 70: 27–30, 2003. © 2003 Wiley-Liss, Inc.