A multicentre study was undertaken in order to determine the reliability of the methods of assay of F VIII:C and the position of the peak value obtained after infusion of F VIII:C concentrates. Blood samples were drawn before and 10, 30 and 60 min after injection of F VIII concentrates in six haemophiliacs in one of the centres. Coded, frozen plasma samples were dispatched to the laboratories of the other four centres. F VIII:C was determined by different one-stage methods using the same international standard but with different activators. The results of the different laboratories differed widely and no agreement was reached on the existence of a lag period. To reach a valid conclusion not only the same sample has to be analysed, as in this study, but also the same laboratory technique has to be used by all participating investigators. To reach agreement on in vivo recovery and on elimination curves for different F VIII concentrates multicentre studies must be based on reliable methods of assay.