The effects on mouse skin of varying doses of light from a high-voltage mercury lamp were compared to the alterations occurring after repeated light applications from a carbon-arc xenon sunlamp. A single dose of UV light at a carcinogenic wavelength caused necrosis and ulceration, followed by scarring, but failed to produce tumors. In the case of repeated small doses, the neoplastic response was predominantly epithelial, resulting in formation of papillomas and squamous cell carcinomas. Increasing the dose caused formation of fibromas and fibrosarcomas of the ear. However, repeated applications of UV light having a spectrum similar to sunlight failed to induce tumors. In these studies, only animals showing initially destructive lesions ultimately presented tumor formation, while the hyperplasia induced by UV irradiation showed no relationship to neoplastic transformation. The results emphasize the significance of dose and wavelength in evaluating effects of UV-light skin carcinogenesis.