Previous observations and experiments have shown that the potent carcinogenic wavelengths for skin cancer in man are found in the 290- to 320-nm range, although shorter and longer wavelengths can also have an effect. The short wavelength limit (290 nm) is determined by thesolar emission reaching the earth's surface, a parameter that varies greatly with season, time, and atmopheric conditions. The long wavelength limit (320 nm) was based on observations of the effect of window glass filtration and on comparison of the effect of mid and long UV radiation. It provide little information as to the efficacy of wavelengths in this range. We performed a series of experiments to provide more specific comparative data by testing the hypothesis that the action spectrum for carcinogenesis parallel that for erythema. Albino mice were exposed the emission of a diffraction grating monochromator (5 nm half power bandwidth) at 290, 300, 310, and 320 nm. Energy levels were proportional to valves for the erythema spectrum of untanned Caucasian human skin. Exposures were given thrice weekly until 50% of the mice had developed tumors. Squamous cell carcinomas developed at approximately the same rate and frequency when the UV exposure was proportional to that for erythema, which indicated a decreasing potency from 300 to 310 to 320 nm. No tumors occurred in mice exposed to 290 nm.