This is a double-blind, placebo-controlled, research study to evaluate the effectiveness and safety of the experimental compound OP2000 (deligoparin) in patients with active ulcerative colitis. Patients eligible for this study will have received (and will continue to receive) stable doses of aminosalicylates (oral, enema and/or suppository), if tolerated. OP2000 is an ultra low molecular weight heparin with anticoagulant (blood thinning) and anti-inflammatory actions that may be of benefit for the treatment of ulcerative colitis. Patients will be randomized (assigned by chance like the toss of a coin) to receive 75 mg OP2000, 125 mg OP2000, or placebo once daily for 6 weeks. Study drug will be administered by subcutaneous (under the skin) injection and patients will be taught how to self-administer these injections. Following an initial Screening Visit, eligible patients will return to the clinic for initiation of study treatment and then again for follow-up visits after 1, 2, 4 and 6 weeks of treatment. A follow-up telephone call will be scheduled 2 months (and possibly also 4 and 6 months) after completing study treatment. Study procedures will include a flexible sigmoidoscopy at the Screening Visit and at the Week 6 visit.