The authors have evaluated the use of heparin as a method to prevent postoperative intraocular fibrin clot formation in the rabbit after vitrectomy and cyclocryotherapy. In addition, they have studied the effect of a heparin infusion on intraocular bleeding after sectioning of retinal vessels. Heparin was administered by several different routes. The extent of the postoperative fibrin clot, as well as the number of days until its clearing, were recorded. A single anterior chamber injection, heparin supplementation of the infusion solution, or a single intravenous (IV) injection, all resulted in a statistically significant reduction of postoperative intraocular fibrin. Once daily subcutaneous injections alone did not produce a reduction in postoperative fibrin. No ocular bleeding complications developed postoperatively. A constant heparin intraocular infusion of 10 IU/cc did not change the bleeding time after sectioning of a retinal vessel.