In this study, patients with moderate to severe rheumatoid arthritis who are being treated with methotrexate will receive 2 intravenous treatments with either PF-05280586 or Rituxan (Rituximab) or MabThera (Rituximab). During the course of the study, the effects of the drugs will be assessed by sampling the levels of drug in the blood, blood cell counts, and by comparing these levels among the different treatments. Safety, tolerability and immunologic response also will be evaluated throughout.