Chromosome aberrations were scored in 48-h leukocyte cultures from pigs subjected to whole-body or half-body gamma irradiation with 100, 150, 200, 300, or 400 R. Half-body irradiation resulted in the recovery of approximately half as many aberrations as did equivalent whole-body exposures at levels of 200 R or less. Higher exposures yielded proportionally fewer anomalies in half-body irradiated subjects. These lower levels apparently resulted from the selective disadvantage of irradiated cells in coming to mitosis but did not seem to be related to the amount of chromosome damage sustained by the cell. When adjustments were made for effective dose to the in vivo cells, the dose-response pattern showed good agreement with published values for mixtures of normal and in vitro-irradiated human lymphocytes.