The Aims of this study were (a) to determine the dose-lethality of the tamarin to gamma irradiation and (b) to sequentially evaluate the changes in the peripheral blood cells resulting from various levels of whole-body irradiation utilizing slightly higher energy than that employed by the single preceding writer (2). Five groups of 4 animals each, of mixed sexes, weighing between 240 and 490 g, and selected for exhibiting an optimal condition of health, were exposed to a single, whole-body dose of cobalt-50 gamma radiation from the clinical teletherapy unit. Precautions were taken to correct variations in exposures due to the single-beam geometry, employing plastic chambers which revolved within the primary beam. Calibration and the dose rates were established with rice phantoms of appropriate shape and volume, and a secondary standard ionization chamber was placed in a phantom prior to exposing the animals. Each group received a single whole-body exposure ranging from 100 to 500 rads with an increment of...