The transdifferentiation of pigment epithelial cells in vitro was reviewed on the basis of results in the classic studies of lens regeneration in the newt. It is now known that pigment epithelial cells of avian embryos and human foetuses as well as of the newt can readily transdifferentiate into lens cells, when dissociated and cultured in vitro. The evidence now permits us to postulate that the potential of the pigment epithelial cell population for transdifferentiation is strongly conserved in vertebrates in general. Several environmental conditions controlling the processes have been suggested, and we can now provide an in vitro experimental system which is particularly useful for analysis of the molecular basis of transdifferentiation of the pigment epithelial cell, and its role as the cellular basis of lens regeneration.