Abstract.Factor VIII‐related properties (coagulant = VIII: C, ‘Willebrand’ factor = VIII R: WF, antigen = VIII 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 VIII R: AG/V1II R: WF. The relative increase of VIII R: AG, and/or loss of VIII 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