The goal of the collaborative study was to calibrate the B19 DNA content of a candidate Biological Reference Preparation (BRP) that is intended to be used for the validation of the analytical procedure, as threshold control and/or as quantitative reference material in the Nucleic Acid Amplification Technique (NAT) test of plasma pools for detection of B19 contamination. The candidate BRP was calibrated against the 1st International Standard for B19 DNA NAT assays. According to the European Pharmacopoeia monograph Human anti-D immunoglobulin, the threshold control needs to have a titre of 10( 4) IU/ml of B19 virus DNA. The lyophilised candidate BRP was prepared from 0.5 ml aliquots of a plasma pool spiked with B19 virus. The B19 virus originated from a "B19 virus window phase" blood donation (anti-B19 negative, B19-DNA high titre positive) and was diluted in a plasma pool tested negative by both serological and NAT assays for Hepatitis B Virus, Hepatitis C Virus and Human Immunodeficiency Virus 1 to obtain a B19-DNA concentration level in the range of 10( 6) copies/ml. The residual water content of the lyophilised candidate BRP was determined as 0.98 +/- 0.65% (mean +/- relative standard deviation). Sixteen laboratories (Official Medicine Control Laboratories, manufacturers of plasma derivatives, NAT test laboratories and NAT kit manufacturers) from nine countries participated. Participants were requested to test the candidate BRP and the International Standard (99/800) in four independent test runs on different days using their in-house qualitative and/or quantitative NAT methods. Sixteen laboratories reported results. Thirteen laboratories reported results from qualitative assays and 5 laboratories reported results from quantitative assays. Two laboratories reported results from both types of assay. For the qualitative assays a weighted combined potency of 5.64 log( 10) IU/ml with 95 per cent confidence limits of +/- 0.17 log( 10) which corresponds to 67 to 150 per cent of the estimated potency was determined. For the quantitative assay the semi-weighted combination of the 5 potency estimates lead to a combined potency of 5.83 log( 10) IU/ml with 95 per cent confidence limits of +/- 0.04 log( 10) which corresponds to 91 to 110 per cent of the estimated potency. The semi-weighted combination of the results from both types of assay lead to a final potency estimate of 5.80 log( 10) IU/ml with 95 per cent confidence limits of +/- 0.05 log( 10) which corresponds to 86 to 117 per cent of the estimated potency. A threshold control suitable for quantitative testing of B19 DNA and reflecting the requirements of the European Pharmacopoeia monograph Human anti-D immunoglobulin should contain 104 IU/ml of B19 DNA. Taking into account the assigned titre of 5.80 log( 10) IU/ml (630.957 IU/ml), a dilution of 10(-1.8) (1/63) of the candidate BRP should yield a positive response. This concentration was easily detected by all participating laboratories in all tests. The candidate BRP thus appeared to be suitable for the intended purpose. The candidate BRP was adopted as the European Pharmacopoeia BRP by the European Pharmacopoeia Commission at its session in June 2003 and is available from the European Directorate for the Quality of Medicines (Catalogue Number: Y0000285).