Mice were exposed to 800 R whole body X-irradiation at a rate of 15 R per minute. Following exposure there was an increase in the permeability of L-alpha hydroxy acid oxidase from mouse liver peroxisomes, and an increased sensitivity of the peroxisomes to low concentrations of Triton X-100, These results indicate that latency characteristics of peroxisomes from irradiated animals are altered compared to the latency characteristics of peroxisomes from control animals.