BACKGROUND AND OBJECTIVES: A collaborative study, involving 26 laboratories from 14 countries, was carried out in order to establish a World Health Organization (WHO) International Standard for human parvovirus B19 (B19) DNA nucleic acid amplification techniques (NAT). MATERIALS AND METHODS: Four samples: AA, BB (which were lyophilized), CC and DD (which were liquid preparations) were analysed using several different NAT assays. The mean B19 DNA content of each sample was determined for each laboratory using an end-point dilution method. RESULTS: There was good agreement between the overall mean 'equivalents'/ml obtained by the different assays. The mean log(10) 'equivalents'/ml were 5.76 for sample AA, 5.73 for sample BB, 5.82 for sample CC and 7.70 for sample DD. The differences in titre among samples AA, BB and CC were not statistically significant, but the titre of DD was significantly higher. CONCLUSIONS: Despite the range of NAT assays used in the study, it was possible to calculate the mean B19 DNA concentrations in the four preparations. Lyophilized preparation AA was established as the first International Standard for B19 DNA NAT assays and was assigned a concentration of 10(6) international units (IU)/ml.