Fibrinogen was injected in the vitreous cavity of 29 rabbits with follow-up for 75 days. Vitreous membrane formation was detected by indirect ophthalmoscopy and confirmed by histopathological study. Membrane formation was significantly more common in the fibrinogen group than in the control group. Fifteen days after injection, the membranes decreased in size and gave place to vitreous liquefaction in the majority of eyes in the fibrinogen group. Histochemical stains for fibrinogen were positive in half of the vitreous membranes. Fibrinogen is thought to be transformed into a long fibrin polymer forming a matrix for surrounding cells to proliferate in the vitreous. An effective fibrinolytic system in the vitreous explains the ultimate resolution of most of the fibrinogen-induced membranes. Simple fibrinogen injections do not provide a good model for long-term vitreous band formation, yet they add more evidence to the role of fibrin in the pathogenesis of vitreoproliferative diseases. The spontaneous resolution of some proliferative vitreoretinopathies parallels the reversibility of most of the fibrinogen-induced vitreous membranes.