Compare anti-ADA antibody formation and serum ADA levels in patients on ADA compared with those on combination ADA and MTX. ADA is a fully human monoclonal antibody to TNF-a. More recent advances in detection of ADA antibodies have shown that many patients develop antibodies to ADA while on treatment and some of these antibodies are neutralizing. The presence of anti-ADA antibodies has been correlated with loss of therapeutic response in many disease states.