Although various growth factors have been identified in the human aqueous humor, their sources have not been fully established so far. To determine, whether the ciliary body epithelium is capable of producing growth factorsin vivo, we studied the immunolocalization of EGF, bFGF, IGF-I, TGF-α, TGF-βand PDGF-AB in human ciliary body tissue obtained from 20 autopsy eyes (12 to 88 years; fixed within 6 hours post mortem) and 1 surgically enucleated melanoma eye using light and electron microscopic immunohistochemistry. Antibody binding was visualized by indirect immunofluorescence and immunogold labeling on differently fixed frozen and resin-embedded sections. The immunohistochemical findings indicate the production of EGF, bFGF, IGF-I, and TGF-α, to a minor degree also TGF-β, particularly TGF-β2, by the ciliary epithelial cells, predominantly the nonpigmented cells. Ultrastructural evidence for an endogenous production included the distinct and specific labeling of secretory organelles (rough endoplasmic reticulum, Golgi complex), cytoplasmic vesicles, and the basolateral membrane infoldings. The ciliary epithelium failed to stain significantly with antibodies to TGF-PI and PDGF-AB. Labeling for bFGF was found to depend on the specific antibodies and fixation conditions employed. Sequestration of bFGF and PDGF-AB in the basement membranes of the ciliary epithelium could be demonstrated under certain conditions. Peaks of labeling intensity were consistently observed at the crests of the ciliary processes and in the pars plana, suggesting regional variations in activity and secretion of growth factors into the aqueous humor and vitreous. While only PDGF-AB may be derived from the serum, it appears likely that most growth factors demonstrated are not circulating hormones but rather act as autocrine and/or paracrine factors. Curr. Eye Res. 12: 893-905, 1993.