In vivo studies of mice were performed to investigate whether auto-reactive antibodies specific for self CD20 antigen on B cells could be induced by immunizing with a CD20 peptide linked to a foreign, human IgG.Fc fragment through a T cell immunologically inert linker peptide and how such an auto-reactivity, if generated, would affect the levels of B cells. The dimeric Fc fusion protein containing the extracellular 44-amino acid portion of CD20, and the CH2-CH3 domains of human gamma 1 immunoglobulin were prepared. After several subcutaneous immunizations with this CD20-Fc protein, mice produced anti-CD20 antibodies that can bind to native CD20 on normal B cells and B-lymphoma cells. In mice immunized with the CD20-Fc protein, the fraction of B cells in total peripheral blood lymphocytes decreased to about 40%, significantly lower than that of mice immunized with human IgG. In addition, antibody response towards an irrelevant bystander antigen, chicken ovalbumin, was weakened compared with that of mice immunized with human IgG. These results show that auto-reactive antibodies specific for CD20 can be induced by immunizing with an autologous CD20 peptide fused with a foreign IgG.Fc and that the auto-antibodies can partially reduce the levels of B cells and their response to other antigens.