Factor VIII was purified 1200-fold from commercial concentrates (Centre National de Transfusion Sanguine) by immunoaffinity chromatography using an anti-(80-kDa light chain) monoclonal antibody. The different molecular forms isolated were subsequently separated and analyzed using Fast Protein Liquid Chromatography and sodium dodecyl sulphate polyacrylamide gel electrophoresis analysis. The different factor-VIII forms obtained, consisted of 80-kDa light chains, each being associated with one more-or-less fragmented heavy chain ranging over 90-210 kDa. The specific activity of these different forms was 7000 U/mg. At different stages of activation of factor VIII by thrombin, various forms were separated and identified. Activated complexes were found to result from the association of the 70-kDa light chain (generated from the 80-kDa light chain) with heavy chains ranging over 90-210 kDa. Two different thrombin activation steps were characterized. The first step corresponding to the cleavage of the 80-kDa light chain led to a sixfold increase in the procoagulant activity, and yielded a stable activated intermediate form. Compared with normal factor VIII, the ratio of von Willebrand activity to factor-VIII activity, measured in the activated fractions, decreased, indicating that von Willebrand factor dissociates from factor VIII after proteolysis of the light chain by thrombin. In the second step, the 90-kDa heavy chain was cleaved into two polypeptides of 45 kDa and 50 kDa, which were associated with the 70-kDa proteolyzed light chain, generating the final activated complex (45-50-70 kDa). The new intermediate forms we described imply a new scheme for the multistep activation process of factor VIII.