We report a method for rapidly inducing cancer in the hairless mouse utilizing regimen in which an exposure to highly erythemogenic, but otherwise clinically noninjurious, dose of broad spectrum (290-400 nm) ultraviolet light is increased by 20% every 6th day. Clinical and histological observations reveal the presence of squamous cell cancer after as little as 18 days of irradiation. The rate of cancer induction is enhanced by the 320-400 nm component and this enhancement is shown to be a photoaugmentative effect. The results support the idea that stratum corneum and/or malpighian layer thickening produced in early stages of tumor induction tends to protect against the detrimental effects of UV radiation. Strict monitoring of both the spectral distribution and output of the radiation source is imperative for reproducible rates of tumor induction.