Rituximab is a monoclonal antibody widely used in the treatment of malignant lymphoma and autoimmunity. Its epitope within the B-cell antigen CD20 is largely unknown. We used phage display libraries to select peptides binding to rituximab. Enriched peptides showed 2 sequence patterns: one motif (CALMIANSC) is related to (170)ANPS(173) within CD20, while another motif (WEWTI) may mimic the CD20 segment (182)YCYSI(185). Phages displaying either motif specifically bound rituximab. Binding to rituximab by the CD20 peptides ANPS and YCYSI was weak when used separately and enhanced when both peptides were linked. Recombinant CD20 extracellular loop proteins blocked binding of the selected CWWEWTIGC phage to rituximab, suggesting that CWWEWTIGC mimics the epitope. Blocking capacity was strongly reduced upon mutation of the CD20 strings ANPS or YCYSI. We conclude that rituximab binds a discontinuous epitope in CD20, comprised of (170)ANPS(173) and (182)YCYSI(185), with both strings brought in steric proximity by a disulfide bridge between C(167) and C(183).