Rituximab is a chimeric monoclonal antibody to the surface antigen CD20 and has provided better outcomes against CD20+ B-cell lymphomas than chemotherapy with conventional antitumor drugs alone. Treatment with rituximab poses a considerable problem, however, because of CD20- tumor transformation and subsequent disease progression. We have established a CD20- lymphoma cell line, RRBL1, from a diffuse large B-cell lymphoma with CD20- transformation from CD20+ follicular lymphoma after treatment with rituximab. RRBL1 was CD10+, CD19+, and CD20- by flow cytometry. CD20 expression was not detected by immunohistochemistry. Immunoblotting with whole RRBL1 cell lysate showed a very faint CD20 band only with longer exposures. The level of CD20 messenger RNA (mRNA) expression detected by quantitative reverse transcriptase-polymerase chain reaction analysis was almost 100 times lower than that in CD20+ lymphoma cells. When we treated RRBL1 cells with trichostatin A, an epigenetic drug that modulates histone-acetylation status, we detected dramatically increased CD20 mRNA and protein expression, suggesting that epigenetic mechanisms may explain the CD20- phenotype in RRBL1 cells. Thus, RRBL1 may be useful not only for analyses of mechanisms for the absence of CD20 expression in vitro but also for exploration of therapies against CD20- B-cell malignancies in vivo.