The purpose of this study is to evaluate the safety of and the clinical response to a human monoclonal anti-interleukin-12 antibody (J695) administered to patients with Crohn's disease. Crohn's disease, an incurable, chronic, relapsing inflammation of the small and large intestine, affects approximately 500,000 people in the United States. The disease is characterized by full-thickness involvement of the gut wall leading to episodes of abdominal pain, diarrhea, hematochezia, weight loss and complications such as bowel obstruction, fistula formation and extraintestinal manifestations. The rationale for this study is based on two compelling observations. First, despite the standard therapeutic use of steroids, aminosalicylates, antibiotics, antimetabolite immunosuppressants (6-MP, methotrexate), and early agents of the emerging biologics class of drugs (anti-TNF alpha antibodies, e.g.), the treatment of Crohn's disease is still troubled by loss of effectiveness of standard therapy over time, outright nonresponsiveness, and serious medication side effects. For these reasons newer agents for the treatment of Crohn's disease need to be developed and tested. Second, studies in both animals and humans support a central role for IL-12 in the gut inflammatory response in Crohn's disease. Administration of anti-IL-12 antibodies in animal models has prevented inducible or ameliorated established colitis. For these reasons IL-12 is an appropriate therapeutic target for the treatment of Crohn's disease.