Proportionate twofold or greater increases in both Factor VIII procoagulant activity (VIII AHF) and Factor VIII-like antigen (VIII AGN) have been observed to follow the administration of adrenalin in normal individuals. Levels of factor VIII procoagulant or clot-promoting activity (VIII AHF) and VIII AGN were measured immediately before and one hour after the subcutaneous injection of adrenalin (0.35 ml) in ten adults with hemophilia A documented by low VIII AHF levels (ranging from 0.01-0.25 units/ml, normal VIII AGN levels, and normal VIII VWF levels as measured by the washed-platelet ristocetin assay). Eighteen normal adults served as controls. Results were analyzed by the paired test. It was shown that VIII AGN increased significantly, while VIII AHF did not change, in hemophiliacs after adrenalin stimulation, regardless of the base line level. This fixed rate of VII AHF production, release, or activation may help to explain the constancy of VII AHF levels in individual hemophiliacs and their affected family members.