Explants of epithelial cells from newborn rat lenses undergo changes characteristic of fibre differentiation when cultured with neural retina or retina-conditioned medium. Here we show that similar changes occur when acidic and basic fibroblast growth factor (FGF) are used instead of retina-conditioned medium. When cultured without FGF, epithelial explants contained negligible amounts of beta-crystallin, a lens protein found only in fibre cells. However, at saturating concentrations of FGF, about 20 micrograms beta-crystallin was produced per explant in 5 days. The response was dose-dependent, half maximal response requiring 55 and 290 ng/ml of basic and acidic FGF, respectively. FGF also stimulated cell proliferation and cell migration. All three responses to basic FGF were blocked by an antibody specific for basic FGF. The concentration of FGF required to produce a maximal response was lower for cell proliferation and migration than for beta-crystallin accumulation. The results suggest a possible role for FGF in the control of events in lens development.