In a preliminary investigation into the origins of the extracellular matrix glycoprotein fibronectin in proliferative diabetic retinopathy (PDR) membranes, four globes containing PDR membranes were subjected to in situ hybridisation and immunohistochemical studies. Fibronectin mRNA was found in PDR membrane cells in two of the four globes. The fibronectin-producing cells exhibited fibroblast-like or macrophage-like morphology. Occasional intraretinal macrophage-like cells which contained fibronectin mRNA were also observed in three of the eyes. Variable fibronectin immunoreactivity was observed in all the PDR membranes, while the fibronectin staining pattern in the underlying retina was similar to that reported for albumin in eyes with breakdown of the blood-retina barrier. The findings suggest that fibronectin in PDR membranes originates from both plasma-derived fibronectin and the membrane cells, and support the concept that extracellular matrix components play an important role in PDR membrane progression.