Proliferative vitreoretinopathy accounts for most of failures in retinal detachment surgery. It results from the formation of membranes spreading onto inner and outer surfaces of the detached retina and within the vitreous body, but the nature of the growing cells and the mechanisms of proliferation remain speculative. A cytological study was thus undertaken on 35 specimens of vitreous and subretinal fluid obtained surgically in patients with proliferative vitreoretinopathy. Various types of cells were identified: typical pigment epithelial cells, lightly pigmented and large totally unpigmented macrophage-resembling cells, smaller unpigmented cells and lymphocytes. Immunocytological procedures with 10 different monoclonal antibodies directed against different markers of epithelial and immunocompetent cells showed the epithelial nonmacrophagic origin of the intravitreal and subretinal cells, as most of these cells were positive for cytokeratin but remained negative for macrophage markers. Examination of intravitreal pigment granules, using autofluorescence analysis by epi-illumination and toluidine blue staining, showed two distinct populations of pigmented cells, one containing melanin and the other lipofuscin, suggesting that pigmented cells could originate from the retinal and ciliary pigment epithelia. As concerns lymphocyte identification, only B cells were seen, whereas no T lymphocyte could be found. Fibronectin was found on a minority of cells in 4 vitreous specimens, but cells positive for glial fibrillary acidic protein could not be seen. These results confirm the involvement of pigment epithelial cells and the strong morphological changes they undergo during the course of proliferative vitoretinopathy, but the mechanisms of proliferative phenomena after retinal detachment remain to be determined.