We determined hemograms of cattle at least semiannually for 11 or 12 yr after whole-body or sham irradiation. Three groups of female Hereford cattle, 18 months old, were exposed to 200, 300, or 400 R; a fourth group received two 300-R exposures 8 weeks apart, and a fifth group served as controls. One month after the last irradiation, each group contained 40 to 51 animals. Three weeks after exposure, granulocytes and platelets were more depressed than agranulocytes. By 8 weeks postirradiation, platelets in the irradiated animals were similar to those of controls. Granulocytes were still depressed but had recovered to a greater degree than agranulocytes. One year after exposure, granulocyte and platelet counts of irradiated animals did not differ from those of controls. Agranulocyte levels in irradiated groups, however, still showed a highly significant (P < 0.01) depression. The radiation-induced depression of agranulocyte counts was significant (at the P < 0.01 level) through the third year and significant (at the P < 0.05 level) for the next 18 months. At 5 yr after irradiation and for the duration of the experiment, hemograms of irradiated animals could not be distinguished from those of controls. Irradiation did not significantly change the erythrocyte counts or any related parameters at any of the times when these measurements were made.

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