To the Editor: The coagulation laboratory for the Hemophilia Center at Orthopaedic Hospital was established in 1967, and it immediately began clinical testing of factor VIII concentrates, performing factor VIII assays in patients' plasma obtained before and after infusions. The assays were used to verify expected factor VIII recoveries and plasma half-disappearance times while the concentrates were still experimental drugs, and later for clinical surveillance.In vivo recoveries of factor VIII during the early years averaged more than 100 per cent of the expected values.* In recent years we have observed a decline in the observed factor VIII recovery as . . .