A quantitative PCR assay is described for serum HCV RNA which is based on a recombinant competitive titrating RNA template. This template is derived from the 5' non-coding region of the genome and generates a shorter PCR product (93 bp) than that from the serum-derived wild type genomes (279) bp from the same sets of primers. By using this assay we have found serum HCV RNA concentrations ranging between 3.3 x 10(2) and 6.6 x 10(8) viral genomes/ml of serum in 15 samples from 10 different patients, 9 cases of chronic hepatitis and one case of fulminant hepatitis. Comparison of viraemias with serum transaminase levels has shown a good correlation for individual patients between these two parameters during chronic HCV hepatitis.