When 300, 310, or 320 nm monochromatic UV energy was given to albino mice in amounts proportional to its erythemal effectiveness, skin tumors developed on the exposed ears of the mice at approximately the same time and the same rate in all three groups. Thus the long wavelength limit of the carcinogenesis action spectrum paralleled that of the erythema action spectrum. No tumors were found in mice exposed to a 290-nm UV wavelength nor in those exposed to low dosages at 310 nm. All tumors observed in the group exposed to 300 nm were squamous cell carcinomas; 3 sarcomas and 5 squamous cell carcinomas were seen with the more penetrating doses of a 310-nm UV wavelength.