Cell proliferation and membrane formation found in the posterior parts of the eye after production of retinal detachment in owl monkey eyes had either the characteristics of macrophages or of membrane-forming epithelial cells. The proliferating cells originated from pigment epithelium. These membranes caused folding of the retina and strand formation in the vitreous cavity. Since cell proliferation was present on all available surfaces, such as the upper and under surface of the retina, and along vitreous surfaces, we propose to call this entity "massive periretinal proliferation" (MPP).