OBJECTIVE The anticoagulants, unfractionated heparin and low-molecular-weight heparin, demonstrated anti-inflammatory effects in animal models and in humans. Because of its dual effects, high-dose heparin was proposed as a therapeutic modality for ulcerative colitis. We investigated whether a low dose of low-molecular-weight heparin-enoxaparin (Clexane, Rhône-Poulenc Rorer, France)-ameliorates the inflammatory response in two models of experimental colitis. METHODS Colitis was induced in rats by intrarectal administration of dinitrobenzene sulphonic acid. Enoxaparin (40, 80 and 200 microg/kg) or unfractionated heparin (100, 200 and 400 U/kg) were administered subcutaneously immediately after the induction of damage. Enoxaparin, 80 microg/kg, was also administered after induction of colitis by intrarectal administration of iodoacetamide. Rats were sacrificed 1, 3 or 7 days after induction of injury. Colonic damage was assessed macroscopically and histologically. Mucosal prostaglandin E2 generation, myeloperoxidase and nitric oxide synthase activities and tumour necrosis factor-alpha levels in blood were determined. RESULTS Enoxaparin and heparin significantly ameliorated the severity of dinitrobenzene sulphonic acid- and iodoacetamide-induced colitis as demonstrated by a decrease in mucosal lesion area, colonic weight and mucosal myeloperoxidase and nitric oxide synthase activities. The dose-response curve had a bell-shaped configuration: enoxaparin, 80 microg/kg, and unfractionated heparin, 200 U/kg, were the optimal doses. CONCLUSIONS Low-dose enoxaparin and unfractionated heparin ameliorate the severity of experimental colitis. This effect is related to their anti-inflammatory rather than anticoagulant properties.