Comparison of continuous versus intermittent (every 12 h bolus) infusion of a high-purity Factor VIII concentrate in 12 severe hemophilia A patients undergoing surgical procedures indicated that a higher (50 u/dl versus 35 u/dl) minimal plasma level was seen with the same amount of product when given continuously. A continuous VIII concentrate infusion of 2 u/kg/h produces a mean VIIIc level of about 50 u/dl or, alternatively, about 0.04 u/kg/h raises the VIIIc level by 1 u/dl (1%). None of the patients showed any abnormal bleeding. Higher plasma levels of VIIIc were noted by the two-stage VIIIc assay than by the one-stage VIIIc assay. Fibrinogen levels were normal or only slightly elevated, and the monomer test was consistently negative.