Adult grade Hereford cattle were exposed to whole-body Co60 gamma-radiation of 450 to 700 r. The LD50/30 dose was 543 r. Preliminary measurements of dose at mid-line indicated an LD50/30 of approximately 160 r. The mean survival time was 20.3 days. All deaths were associated with depression of hematopoietic tissue and infection. Blood changes were comparable to those reported for other large domestic animals.