Inclusion Criteria:

• Male and female, aged at least 18 years old at the Screening visit.
• A clear chest X-ray within 3 months prior to Baseline visit.
• A diagnosis of adult-onset RA (of at least six months duration but not longer than 15 years prior to Screening) as defined by the 1987 American College of Rheumatology classification criteria.

• Active RA disease at Screening and Baseline as defined by:

  • ≥9 tender joints.
  • ≥9 swollen joints. and fulfilling 1 of the following 2 criteria:
  • ≥30 mm/hour ESR (Westergren), or
  • CRP >15 mg/L.
• Must have received a stable dose of MTX with or without folic acid for at least 3 months prior to Baseline visit. The minimum dose is 10 mg MTX weekly.
• Patient must be willing to attend for a Week 52 X-ray of the hands and feet even if they are no longer receiving study treatment but have not withdrawn their informed consent.

Exclusion Criteria:

• A diagnosis of any other inflammatory arthritis e.g., psoriatic arthritis or ankylosing spondylitis.
• A secondary, non-inflammatory type of arthritis (e.g. OA or fibromyalgia) that in the Investigator's opinion is symptomatic enough to interfere with evaluation of the effect of CDP870 on the patient's primary diagnosis of RA.
• A history of an infected joint prosthesis at any time with prosthesis still in situ.
• Does not meet exclusionary concomitant medication criteria.
• A history of a lymphoproliferative disorder including lymphoma or signs and symptoms suggestive of lymphoproliferative disease at any time.
• Patients at a high risk of infection in the Investigator's opinion (e.g. leg ulcers, indwelling urinary catheter and persistent or recurrent chest infections).