The purpose of this study is to evaluate biosimilarity between test product MabionCD20® and the reference product - MabThera® (rituximab) and to demonstrate comparative safety and tolerability of a single course of treatment of MabionCD20® and MabThera® in patients with moderate to severe active rheumatoid arthritis.

Subjects who meet the criteria for participation in this study receive 2 intravenous infusions of MabionCD20® or MabThera® with two weeks interval in combination with methotrexate and folic acid. After the treatment period, patients are followed for 24 weeks, in order to continue checking safety. Therefore, the total study lasts 48 weeks. There is a possibility to repeat treatment after 6 months of the first infusions if subject meets the retreatment criteria.

Inclusion Criteria:

• Patients with active rheumatoid arthritis with disease duration minimum 6 months prior to screening visit
• Patients who are naive to tumor necrosis factor (TNF) antagonists or any other monoclonal antibody therapies
• Patients who have had an inadequate response to an adequate regimen of methotrexate

Exclusion Criteria:

• History of current rheumatic autoimmune disease other than RA and current inflammatory joint disease other than RA
• Contraindications according to the MabThera SmPC and every serious coexisting diseases which, in the Investigator's opinion, would preclude subject participation
• Positive tests for HIV, hepatitis B surface antigen (HBsAg) Anti-HBc antibody, hepatitis C antibody
• Prior treatment with rituximab, other anti-CD20 mAb, anti-TNF-alpha drug or any other monoclonal antibodies
• Pregnancy or lactation or women planning to get pregnant during the course of the study and/or within 12 months post last study drug infusion
• Allergic reaction or intolerance to rituximab, MabionCD20 or any of their components
• Severe allergic or anaphylactic reactions to humanized or murine monoclonal antibodies or previous treatment of any lymphocyte-depleting therapies