After genetic recombination with murine and human cDNA, monoclonal antibody has been produced in chimeric form. CD20 is a B cell antigen, which can be a target molecule for antibody therapy. Rituximab is an anti-CD20 monoclonal antibody that has an efficacy against B cell lymphomas. Combination therapy with CHOP regimen as well as single therapy will give rise to new treatment strategy for CD20+B cell lymphomas. Mechanism is not clarified, but induction of apoptosis, complement-mediated cell toxicity, and antibody-dependent cell toxicity are considered. Synergy with chemotherapy agents such as CHOP, ICE, DHAP and stem cell transplantation will help the patients who suffer lymphomas, CLL and Waldenstrom's macroglobulinemia. Treatment with rituximab is well tolerated and we must be careful about infusion reaction after administration.