Subclass determination of anti-factor VIII antibodies. Binding of anti-factor VIII antibodies to metabolically labeled A2 domain was performed by immunoprecipitation using subclass-specific MoAbs. Immunoprecipitations were analyzed under reducing conditions on a 10% (wt/vol) SDS-polyacrylamide gel. The results of a plasma sample of patient A from 1992 are shown. Lane 1, total IgG; lane 2, IgG1; lane 3, IgG2; lane 4, IgG3; and lane 5, IgG4. Molecular weight markers are given at the right of Fig 2. No binding to αIgM was found (data not shown).