This study proposes to measure the toxicity and clinical effect of two doses of anti-IL-12 (J695) administered subcutaneously to patients with moderately active Crohn's disease. The primary outcome measure is the rate and severity of adverse events. Secondary outcome measures include J695 pharmacodynamics, incidence of anti-J695 antibodies, and clinical response. A separate substudy at the NIH will measure gut lymphocyte apoptosis and gut and peripheral blood cytokine responses to study drug as well as endoscopic and histologic response to treatment. Our short-term goal is to assure good tolerance of anti-IL-12 treatment regimens in Crohn's disease patients and observe a beneficial clinical response. The long-term goal of this study is to establish anti-IL-12 as an effective alternative with a low risk profile in the therapeutic armamentarium for Crohn's disease.