This paper reviews factors that have been reported to influence photocarcinogenesis in laboratory animals. Such factors include the sensitivity of the test animals, the amount of the ultraviolet radiation (UVR) delivered, the mode of its delivery, and interactions of other radiations or of chemicals in the process of carcinogenesis. New data are presented in these areas: reduction in the size of each unit dose (and thus an increase in dosing frequency) increases the carcinogenic effectiveness of a given lifetime dose; certain inbred strains of albino hairless mice exhibit heritable differences in their susceptibility; several chemicals are known to enhance photocarcinogenesis, but they appear to have so little in common, either structurally or functionally, that they offer limited guidance about which other compounds may be effective in this way. Prevention of long-term UVR effects on skin is a desirable goal; development of personal UVR dosimeters will aid in defining the quantitative nature of the problem; improved sunscreens should provide the means to achieve significant reduction in the incidence of UVR-induced human skin cancer.