The induced activity of the enzyme arginase was measured in diploid wild type Saccharomyces cerevisiae after X-ray and 241Am-alpha-particle exposure. It was found that after doses which are comparable to those necessary to reduce survival, little effect on enzyme activity is seen immediately after irradiation but it is reduced on further incubation. In this case there is an oxygen enhancement ratio of about 2 as for survival. Suppression immediately after exposure requires considerably higher doses, and no oxygen effect is seen. To achieve the same effect with alpha-particles requires even higher doses, the apparent r.b.e. is about 0 . 1. X-ray damage to induced enzyme activity is subject to liquid holding recovery. The results are discussed in relation to current theories of gene inactivation by ionizing radiation.