In a double-blind controlled study of 151 patients admitted to a small coronary care unit, the addition of 1,000 units of beef-lung heparin to each liter of fluid given intravenously was found to reduce the frequency of thrombophlebitis at infusion sites. In 34 patients, 5% dextrose in water containing heparin was infused for a total of 2,243 hours with local inflammation requiring a change in catheter location on three occasions. In 30 patients receiving 5% dextrose in water without heparin, infused for a total of 2,659 hours, 20 changes in catheter location were required. Similar but less striking improvement was associated with heparin administration via catheters receiving 5% dextrose in water plus other medications. Among patients receiving conventional therapeutic doses of heparin by a continuous intravenous drip, infusion phlebitis was rarely present clinically, and polyethylene-catheter viability was greatly prolonged.