Albino mice exposed to 1 X 10(-4) W cm-2 of UV radiation in the range 290-400 nm for 12 hr per day developed anterior lens opacities within 1-2 months after onset of the exposure. Histological changes demonstrated marked derangement of the lens epithelium. When the radiation was filtered with glass, eliminating radiation shorter than 320 nm, these opacities did not develop and the histology of the lenses was normal. We conclude that following chronic exposure, UV-B (290-320 nm) radiation is cataractogenic and acts via a mechanism involving the lens epithelium. UV-A (320-400 nm) does not appear to be cataractogenic, even with chronic exposure.