Proliferation of host cells from around the optic nerve head has recently been implicated in the development of experimental proliferative vitreoretinopathy in rabbit eyes injected with homologous fibroblasts. We used liquid scintillation spectrometry to quantitate the tritiated thymidine incorporation into cells in the vitreous, retina, and optic nerve head following intravitreal injection of 250,000 homologous dermal fibroblasts. Cellular proliferation peaked three days after injection of the fibroblasts. The amount of tritiated thymidine incorporation that occurred three days following injection of irradiated homologous fibroblasts (incapable of cellular division) was not significantly different than that following injection of normal homologous fibroblasts, indicating that host cells were responsible for most of the cellular proliferation. Treatment with fluorouracil or triamcinolone acetonide completely arrested cellular proliferation following injection of normal fibroblasts.