In hemophilia A, up to 30% of patients develop neutralizing antibodies (inhibitors) to factor VIII (FVIII). Treatment of an inhibitor patient with anti-CD20 (rituximab) provided an opportunity to study the humoral immune response to the well defined and constantly administered antigen, FVIII, before therapy and after B cell repopulation. Levels of CD20(+) B cells, inhibitor titers as well as antibody titers to selected antigens and FVIII-specific IgG subclasses were monitored. Inhibitors were absent for 420 days after B cell depletion, whereas antibody titers to other monitored antigens remained constant. No changes of FVIII specific IgG subclass distribution were observed. In order to characterize specific epitopes phage displayed random peptide libraries were screened with plasma before treatment and after recurrence of inhibitors. A peptide corresponding to a dominant amino acid motif selected by phage display bound FVIII-specific IgG1 before and after anti-CD20 treatment and partially restored FVIII activity in vitro. This peptide mimics a conformational epitope in the A2 domain of factor VIII, which is still recognized after inhibitor relapse despite complete B cell depletion and long-term absence of inhibitors. Mapping of specific epitopes on a defined antigen gives further information on mechanisms underlying B cell repopulation after rituximab therapy.