Factor VIII-related properties (coagulant = Vili:C, ‘Willebrand’ factor = VIII R: WF, antigen = Vili R: AG) are measured in a constant proportion in normal plasma and certain preparations of highly purified factor VIII (relative ratios: 0.5-1.5). We tested these activities in some commercial, lyophilised concentrates of factor VIII and found a variable increase of the ratio Vili R: AG/VIII R: WF. The relative increase of VIII R:AG, and/or loss of Vili R: WF, was attributed to variable degradation of factor VIII-related protein(s) which was directly visualised by electrophoresis on 2.75% polyacrylamide gels in the presence of sodium dodecyl sulphate.