The effects of whole-body, single-dose irradiations (100, 300, 500, 800 R 60 Co γ rays) on the activities of isoenzymes of alkaline phosphatase in the serum of rats have been studied. The degree of changes in the intestinal and liver isoenzyme activity was directly dependent on the radiation dose. A marked, time-dependent decrease in the bone isoenzyme activity in serum was found only after 800 R. A computer processing of biochemical data by stepwise discriminant analysis enabled an early diagnosis of postradiation injury of the gut and bone and to classify the irradiated rats according to the radiation dose (biological dosimeter).