Despite numerous studies of the role of mononuclear phagocytes in proliferative vitreoretinopathy, the origin of these cells has remained obscure. Notably, retinal microglial cells have consistently been neglected. Applying double label immunohistology with a set of new cell markers to 37 preretinal traction membranes, we have identified a distinct population of proliferating cells presumably of microglial origin. The identification of microglia relies on positive labels for LN-1, Ricinus communis agglutinin-1, vimentin, HLA-DR, and nucleoside diphosphatase, and negative labels for Leu-M1, Leu-M3, EBM-11, von Willebrand factor, CD22, cytokeratin, and glial fibrillary acidic protein. Microglia are much more prevalent in idiopathic than in traumatic proliferative vitreoretinopathy and insignificant in proliferative diabetic retinopathy. HLA-DR expression was not restricted to pigment epithelium as previously reported but also observed in microglia, macrophages, endothelial and glial cells. The detection of retinal microglial cell proliferation suggests a pathogenetic role of these cells and questions current concepts of the cellular biology of proliferative vitreoretinopathy.