The authors undertook a study to investigate the ability of systemic or heparin supplementation of the vitrectomy infusion solution to prevent formation of postoperative intraocular fibrin membranes. A total of 73 eyes were randomized; 26 served as the control group, 23 received 10,000 IU of intravenous bolus of heparin, and 12 eyes each underwent the vitrectomy with an infusion solution containing 10 or 5 IU/cc of heparin. Intraoperative bleeding, postoperative bleeding, and postoperative fibrin formation were graded. A 10-IU/cc heparin infusion resulted in a statistically significant reduction in postoperative fibrin formation (P = 0.04) but increased intraoperative bleeding (P = 0.02). A trend toward reduced postoperative fibrin formation was noted in the intravenous heparin and 5 IU/cc infusion groups.