We observed the morphologic changes of tissue fragments of retinal pigment epithelium (RPE) of chick embryos in floating culture under the presence of bacic fibroblast growth factor (bFGF). At higher concentration (10 and 200 ng/ml) of bFGF, rolled up RPE sheets grew to develop non-pigmented lumps with the structure like onions, while dissociated RPE cells did not from such structure. Histologically, the onion-like structure consisted of lens-fiber-like cells, positive for anti-crystallin antibody and anti-bFGF-receptor antibody. This lentoid structure was not formed at 1 ng/ml or lower concentration of bFGF. The trans differentiation from RPE cells to lens epithelial cells was dependent upon the concentration of bFGF as well as the proper interaction among RPE cells.