 Subjects with a stable methotrexate MTX dose enter the run-in period in which certolizumab pegol CZP will be administered at a dose of 400 mg 2 injections at Weeks 0, 2, and 4 and at a dose of 200 mg with placebo 1 injection placebo, 1 injection CZP at Weeks 6, 8, 10, 12, 14 and 16. The dose of MTX should be stable for at least 2 months prior to the Baseline visit and will remain stable throughout the trial, unless there is a need to reduce the dose for reasons of toxicity. 