Inclusion criteria:

• Clinical diagnosis of moderate to severe rheumatoid arthritis.
• On stable dose of methotrexate.
• 18 to 80 years of age.
• Less than 275 lbs.

Exclusion criteria:

• Joint replacement surgery within 60 days of the start of drug dosing.
• Intra-articular cortisone injections within 28 days of the start of drug dosing.
• Pregnancy.
• History of severe allergic or anaphylactic reactions.
• Active bacterial, viral, fungal, mycobacterium tuberculosis.
• Positive PPD test.
• History of any opportunistic infection.
• Serious persisting local or systemic infection.
• History of malignancy within the past five years.
• Received any vaccine within 28 days of the start of study drug dosing.
• Joint replacement therapy planned within nine months of the start of study drug dosing.
• Chronic disorders apart from RA affecting the joints.
• Significant systemic involvement secondary to RA.
• COPD, asthma, or other pulmonary disease.
• Received any DMARD other than methotrexate in the 28 days prior to the start of study drug dosing.
• Approved biologic RA therapy during the 28 days or 7 half-lives of the drug prior to the start of drug dosing.
• Investigational drug and/or treatment during the 28 days or seven half-lives of the investigational drug prior to the start of study drug dosing.
• Liver disease or abnormal hepatic function.
• Serum creatinine level > 1.5 mg/dL.
• Platelet count < 125,000 cells/mm3.
• WBC count < 3,500 cells/mm3.
• Seropositive for hepatitis B surface antigen.
• Seropositive for hepatitis C antibody.
• Known seropositivity for HIV.