The effect of ultraviolet (UV) radiation on skin was studied in mice. Inbred hairless-mice were irradiated with UV radiation from a solar simulator or monochromator in the wavelength range of 290 to 400 nanometers (nm) 5 days a week for 4 to 14 weeks. Initial exposures were 0.5 to 0.9 minimal erythemal dose (MED) and were increased by 20 percent every 2 weeks. The animals were observed for skin changes. Action spectra (plots of reciprocal MED versus wavelength) were constructed. Tumors appeared as early as 4 weeks after starting exposure. Early precancerous responses had action spectra similar to those of mild erythema. Later skin responses showed dose/dependent shifts to longer wavelengths. No tumors occurred at wavelengths below 300nm. Decreasing filter thickness (simulating decreasing ozone concentration) lowered the threshold dose required for tumor production. Mice were irradiated with 280 to 320nm UV radiation 5 days weekly for 4 to 14 weeks, initial exposure 0.9 MED and increasing by 20 percent every other week. Samples of full thickness epidermis, including the stratum corneum, were analyzed for histological changes. Skin absorption spectra were obtained. Exposures of 7.2 MED induced stratum corneum thickening over the range of 280 to 320nm. At higher exposures, thickening progressed markedly for wavelengths greater than or equal to 300nm. Commercial sunscreen preparations containing Padimate-O and benzophenones were applied to the skin of mice. The mice were irradiated as before. The skin was inspected at selected times for pathological changes. After 12 weeks of irradiation, mice with no sunscreen developed raised palpable keratotic plaques, papillomas or tumors. Mice with sunscreen developed only erythema, sometimes followed by scaling. The authors propose a dose/response model for skin thickening by UV radiation. The model predicts a linear relationship between increase in skin absorbance at 320nm and cumulative dose.