Using an electron storage ring as a source of radiation, the wavelength dependence of inactivation and membrane damage in yeast cells (Saccharomyces cerevisiae) was investigated in the range from 145 to 254 nm, with special reference to the effects of vacuum-uv radiation. The cells were irradiated on a Millipore filter in a moist chamber filled with water vapor (deoxygenated) at saturation pressure. Fluence-survival curves taken at 5-nm intervals were generally sigmoidal. Action spectra of the two types of effects were nearly identical in shape. The maximum occurred in both spectra at 160 nm, decreasing sharply toward 180 nm. The spectra remarkably resembled the calculated absorption spectrum of (liquid) water in the range from 145 to 170 nm; the spectra had no similarity at all to the absorption spectra of DNA, proteins, or lipids. These data support the theory that inactivation of wet cells by vacuum-uv radiation may be attributable to damage in the cell membrane initiated by the absorption of water molecules. Above 210 nm the spectrum for inactivation paralleled the absorption of DNA. Genetic changes (induction of gene conversion) were also observed above 210 nm. Photoreversion for the induced convertants was detectable only above 220 nm. These characteristics are consistent with the expectation that above 210 nm the site of major lethal damage shifts to DNA.