To investigate the role of the intermediate filament protein vimentin in the normal differentiation and morphogenesis of the eye lens fiber cells, we generated transgenic mice bearing multiple copies of the chicken vimentin gene. In most cases, the vimentin transgene was overexpressed in the lenses of these animals, reaching up to 10 times the endogenous levels. This high expression of vimentin interfered very strongly with the normal differentiation of the lens fibers. The normal fiber cell denucleation and elongation processes were impaired and the animals developed pronounced cataracts, followed by extensive lens degeneration. The age of appearance and extent of these abnormalities in the different transgenic lines were directly related to the vimentin level. Electron microscopic analysis revealed that the accumulated transgenic protein forms normal intermediate filaments.