Autotransplanted RPE cells after free intravitreal injection or in millipore diffusion chambers placed into the vitreous cavity undergo metaplasia: they first show the characteristics of macrophages. Later they transform into spindle-shaped cells or even into pigment epithelium-like cells. The latter two cell types have definite epithelial cell characteristics, such as cellunctions and basement membrane formation, clearly indicating that they are of ectodermal origin. These results support the hypothesis that RPE proliferation and metaplsia may play a major role in the formation of collagen-containing membranes, such as are found in massive periretinal proliferation (MPP).