Twenty-six laboratories from 10 different countries participated in a collaborative study to establish the 1st International Standard for HIV-1 RNA for use in nucleic acid-based techniques (NAT). Three candidate preparations were tested all based on genotype B viruses. The candidates were tested by each laboratory at a range of dilutions in four independent assays and the results collated and analysed statistically. All three candidates gave results that were tightly grouped, with little difference between the results from different laboratories or from the use of different assays. Studies of relative potency showed good agreement between laboratories. There were no significant differences between five commercial assay types, except that candidate XX showed a slightly lower potency compared to YY and ZZ with a single commercial assay. The reason for this was not established. Degradation studies showed that the freeze-dried preparations were stable at -20,4 and 20 degrees C for 26 weeks, the longest period studied, but that they became difficult to reconstitute after 3 weeks at 45 degrees C and 9 weeks at 37 degrees C. As a result of the study, the World Health Organisation (WHO) Expert Committee on Biological Standardisation (ECBS) established the preparation referred to as candidate YY (NIBSC Code No. 97/656) as the 1st International Standard for HIV-1 RNA for use with NAT with an assigned potency of 100000 International Units per vial.