A test for the immediate determination of the amount of damage done to the leucocytes during irradiation of a patient with large doses of deep x-rays is described. Before and after irradiation a small amount of blood is taken from the finger and mixed with an emulsion of staphylococci. After incubation the average number of staphylococci taken up per leucocyte is noted. The result of irradiation, according to the site irradiated and the dose size, is a fall of 30-50% in this phagocytic index. In cases already severely anemic or actively septic, but in whom it is necessary to give relatively large doses, this test is found to be a valuable safeguard against producing too great a fall in the white cell count. Preliminary experiments suggest that it is advisable to regulate the dosage so that the index does not fall more than 25-30% during any particular irradiation.