SummaryA collaborative study of the first Belgian Standard for Factor VIII assays has been carried out. It involved participants working in five University Hospitals, who were asked: 1) to calibrate the Standard in VIII: C International Units; 2) to measure the VIII: C and VIII R: Ag levels in numerous samples of fresh normal plasma, against the Belgian Standard; and 3) to define whether the Belgian Standard could be used for VIII R: RCF measurements.One VIII R: Ag Unit was defined as the amount of antigen present in that volume of average fresh normal plasma containing 1 VIII: C International Unit.The calibration of VIII: C in International Units disclosed important differences between the five laboratories, probably due to the difficulties encountered in the use of the International Standard. When tested against the 2nd International Standard, the Belgian Standard was calculated to contain 0.99 VIII: C I.U. and 1.08 VIII R: Ag/vial; when tested against average fresh normal plasma taken as 100%, the Belgian Standard was shown to contain activities equal to 78 % for VIII: C and 85 % for VIII R: Ag.Though VIII R: RCF assays are feasible with the Belgian Standard, its direct use as a reference material is not recommended, as the precise VIII R: RCF potency and stability are presently unknown. However, the Belgian Standard could be used indirectly for the calibration of the local plasma pools utilized for VIII R: RCF measurements.The use of the first Belgian Standard for Factor VIII assays in the clinical laboratories of our country is recommended by the authors.