Lens epithelial cells from newborn rats undergo changes characteristic of fibre differentiation when cultured with rat neural retina or with a soluble mitogenic factor present in calf retina-conditioned medium. Mitogens have been isolated from retina in other laboratories, but have not previously been shown to promote fibre differentiation in mammalian lens. We prepared eye-derived growth factors I and II andα-andβ-retina-derived growth factors from bovine retinas. These factors all promoted lens fibre differentiation in our culture system, as assessed by morphological changes and the appearance of fibre cell-specific crystallins. There is now strong evidence that these retina-derived factors are identical to the acidic and basic forms of fibroblast growth factor (FGF), which is present in a variety of tissues. We found that acidic and basic FGF from rat brain also promoted lens fibre differentiation, suggesting that FGF is the factor from the retina responsible for inducing lens fibre differentiation.