Twenty-seven calves were exposed to whole-body gamma radiation from a Co/sup 60/ source with a skin exposure dose rate of about 6.6 r per minute. The tissue exposure doses at the midline ranged from 150 to 360 r. All except one animal receiving a tissue exposure dose at the midline of 250 a or more developed acute hypoplasia of the bone marrow and a hemorrhagic syndrome and died after 14 to 24 days. One caif exposed to 350 r developed an acute intestinal radiation syndrome and died on the fifth day. Two calves exposed to 150 r developed a severe blood dyscrasia between the twentieth and thirtieth day, from which they recovered. They appeared to be normal by 270 days. The time sequence and extent of the changes in the circulating derivatives of the hematopoietic tissues werc essentially the same as those described in other mammalian species. (auth)