A narrow-band UV light source with emission peaks at 292, 300, 307, 317, and 336 nm was developed and used to irradiate whole cages of hairless mice. The purpose was to obtain experimental information on the action spectrum for photocarcinogenesis and dermal elastosis. Groups of 20 mice were irradiated with 500 J/m2 daily, 5 times per week during one year. The total dose was 130 kJ/m2. All mice irradiated with 292 nm and 300 nm developed squamous cell carcinomas. None in the other groups developed malignant skin tumors. Elastosis was estimated quantitatively. The elastic fibers covered 3% of a representative microscopic section of dermis in the control group. In the groups irradiated with peaks at 336 nm, 317 nm, 307 nm, 300 nm, and 292 nm the corresponding percentages were 6%, 13%, 28%, 32%, and 36%, respectively. The shorter the wavelengths the more pronounced was a subepidermal zone replacing the elastotic tissue to the deeper dermis. This zone stained corresponding to a content of glycosaminoglycans (GAG), sulphated GAG, hyaluronic acid, mature collagen, and new reticulin.