To test whether the Müller's cell, and not the fibrous astrocyte alone, is an active glial element in proliferative diabetic retinopathy, we developed a method using enzyme histochemical analysis for carbonic anhydrase and immunolabelling for glial fibrillary acidic protein on plastic-embedded sections to detect Müller's cells in degenerated retinas. Four postmortem eyes with proliferative diabetic retinopathy were studied. Four features of Müller's cell behavior were observed: (1) Müller's cell nuclei migrated within the retina. (2) They were the principal glial cells in preretinal membranes. (3) Müller's cells formed horizontal bands, in the outer retina, that gave the appearance of subretinal membranes. (4) They formed intraretinal bridges between cystic spaces. In conclusion, the Müller's cell (or its progeny) plays an active role in proliferative diabetic retinopathy.