Background and Objectives: The aims of this study were the establishment of a WHO International standard for HCV RNA for nucleic acid amplification technology (NAT) assays and the determination of the HCV RNA content of the candidate standard. Materials and Methods: Twenty-two laboratories evaluated three candidate materials (two lyophilised, AA and BB, which were derived from the same source and one a liquid preparation, CC). All samples were HCV genotype 1 with a concentration of approximately 105 genome equivalents/ml. The methods used included the Roche Amplicor assay (version 1), Chiron Quantiplex (bDNA) assay, Organon Teknika NASBA assay, Transcription Mediated assay and various in-house assays, using single or nested primers. Results: There was reasonable agreement between the overall mean NAT detectable units/ml obtained by the different assays except for some of the in-house assays using single primers which gave substantially lower estimates. These titres were 5.0 log10 for samples AA and BB and 4.6 log10 for sample CC. Conclusions: Sample AA was accepted as the candidate standard and assigned a titre of 105 international units (IU)/ml. The International Standard consists of a batch of vials each containing 50,000 IU/vial. Preliminary studies indicated that the material is stable at +4°C and +20°C for up to 200 days.