Unfiltered broad spectrum radiation emitted by black light, cool white, and black light blue fluorescent lamps and a sunlamp, is both toxic and mutagenic to L5178Y mouse lymphoma cells when the cells are irradiated in phosphate-buffered saline. The increase in mutant frequency seen after exposure of the cells is linear throughout the range of exposures tested. The linear increase in mutagenesis is observed even at exposure levels which do not cause significant toxicity. To facilitate comparison of the differing rates of mutagenesis derived from exposure-response curves obtained for each light source, we have defined a parameter, joule-equivalent mutagenesis (jem), equal to mutants per 10(5) survivors per joule per square meter. Jem values are calculated using the integrated irradiance of each lamp. Based on jem values, the relative mutagenicity of the various lamps tested (compared with a germicidal ultraviolet lamp) is 3 x 10(-3) for the sunlamp, 1 x 10(-4) for the black light and cool white lamps, and 3 x 10(-5) for the black light blue lamp. The toxic and mutagenic effects of the lamps are in reasonable agreement with their relative spectral output from 290 to 330 nm.