Changes of the total serum activity of alkaline phosphatase (AP) and its bone and intestinal isoenzymes were studied in dogs gamma-irradiated by 3.0 Gy. The activities of AP isoenzymes were determined by means of a heat inactivation-inhibition method. After irradiation the serum AP activities were lower in general. In case of the bone isoenzyme the decrease was most pronounced. The changes of the total AP activity and of the intestinal isoenzyme were less distinct. The i.m. administration of the radioprotective mixture cystamine with mexamine (24 mg/kg + 4 mg/kg) before irradiation led only to a moderate reduction of the radiation-altered AP values in the dogs. The present results indicate that the radioprotective way used here renders only an unsatisfactory protection for large laboratory animals against ionizing radiation.