The recovery and initial half-disappearance rate of factor VIII procoagulant activity (VIIIC) and procoagulant antigen (VIIICAg) were studied in 9 haemophilia A patients following infusion of factor VIII concentrates to varying plasma VIIIC levels. While VIIIC recovery was independent of dose, the VIIICAg recovery varied in a dose-dependent fashion. The excess of VIIICAg relative to VIIIC found in the factor VIII concentrates (VIIICAg/VIIIC ratios of 1.9–3.1) was not observed in plasma samples taken after low level infusions (plasma VIIIC < 1 U/ml) but a significant excess of VIIICAg was observed in higher level infusions. The VIIICAg recovery of post-infusion plasma was not increased by treatment with phospholipase