Yeast [Saccharomyces cerevisiae] cells suspended in distilled water were irradiated with monochromatic 163 nm photons by immersing a specially designed discharge tube into the suspension. This was thought to be a useful means of investigating in vivo effects of radiation-induced water radicals on wet cells in the complete absence of ionic species, since 163 nm photons can dissociate water only via excitation. These experiments showed that the water radicals (excluding eaq-) exerted lethal and genetic (gene-conversion) effects quite potently, and the characteristic protection against these effects was observable when 2-mercaptoethanol or, in particular, p-aminobenzoic acid, a specific scavenger for OH radicals, was added to the medium prior to irradiation. Nearly complete protection from lethal and genetic effects was observed in some cases with p-aminobenzoic acid. These results establish unequivocally that the OH radical, and not the hydrogen atom (H radical), possesses the damaging potency in the cell. Comparisons with .gamma.-ray experiments revealed several differences between 163 nm photons and .gamma. rays in the protective actions of radical scavengers, which may be attributable to reactive species other than OH radicals produced by the .gamma. rays.