1. A method has been presented by which granulocytes can be labeled in vivo with diisopropylfluorophosphate containing radioactive phosphorus. The leukocytes are isolated from blood by dextran sedimentation of erythrocytes and are then treated with gramicidin and lysolecithin to remove remaining red cells. Platelets are removed by differential centrifugation. The isolated leukocytes are placed between two squares of scintillating plastic and counted with a scintillation counter.

2. Leukocytes essentially free of erythrocytes and platelets can be obtained by the method outlined. The efficiency of the plastic scintillation counting method for radioactive phosphorus is about 74 per cent and leukocyte samples obtained from 20 ml. samples of normal blood can be counted with a reproducibility of ±10 per cent.

3. The administration of 2 mg. of diisopropylfluorophosphate either intramuscularly or intravenously is without significant toxic side effects.

4. No evidence has been obtained that the label damages the leukocytes.

5. No evidence has been obtained that the label elutes from leukocytes under the conditions of these studies.

6. Diisopropylfluorophosphate labels granulocytes for a brief period of time following injection. The label is not reutilized after death of the cells.

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