Abstract— Hairless mice were irradiated repeatedly by exposure to unfiltered black-light (FR74T12 PUVA) fluorescent lamps and the time to development of skin tumors was determined. For several groups of animals the treatment variable was the size of the weekly dose. A similar approach had been used previously to determine dose-response characteristics for other ultraviolet radiation emitting sources: a xenon arc solar simulator (with a series of five cut-off filters producing five source spectra), and a fluorescent (FS40T12) “sunlamp”. The median tumor latent period (time period for just more than one half of the animals to develop at least one tumor each) was accurately predicted for all these ultraviolet radiation emitting sources by a mathematical equation incorporating the spectral source description and a spectral weighting function. The weighting function judged most appropriate for ultraviolet radiation-induced photocarcinogenesis was the action spectrum, determined previously, for acute (single dose) skin edema in hairless mice. The mathematical equation assigns no effectiveness to wavelengths greater than 330 nm. There was no evidence for wavelength interaction in the spectral range of 26MW nm. Our data, combined with results of others, lead us to conclude that radiation with wavelength greater than 330 nm has an average relative efficacy (297 nm =1.0) less than 0.0002, and that this efficacy is not detectable with sources in which at least 2% of the UV radiation is in the UV-B range.