Intraocular bleeding is often a major problem during vitrectomy for proliferative diabetic retinopathy and trauma. We have studied the effect of thrombin on the control of intraocular bleeding during vitreous surgery in rabbit eyes. Thrombin (100 units/mL) infusion solution reduced the bleeding time by more than fivefold. Toxicity studies showed no adverse effect on the lens and corneal endothelium. Electroretinogram b-wave amplitudes were normal, but electroretinogram sensitivity was reduced by 0.5 log units in experimental eyes. Thrombin appears to be a potential agent for the control of intraocular bleeding during vitreous surgery.