Twenty-six laboratories including 22 Blood Transfusion Centres in England, Scotland, Wales and Northern Ireland participated in a collaborative study to assess the suitability of proposed working standards for hepatitis B surface antigens (HBsAg) for use in the UK Blood Transfusion Service. Data from 88 assays indicated that all laboratories could detect HBsAg in the proposed working standard at a concentration of 0.5 IU/ml. Samples containing HBsAg at a concentration of 0.125 IU/ml were scored positive in 80% of assays and it was concluded that this would be a suitable concentration for inclusion in a monitor sample which would indicate whether the sensitivity of an assay system in routine use is varying over a period of time. The use of a batch of 'provisional' working standard tested under field conditions over a period of 4 months has been monitored. HBsAg was not detected in only 2 out of 6,523 assays.