18 H. MAISIN 



We also gave 10 r and 5 r but we were unable to induce radioresistance. 

 However, 50 r whole-body irradiation increases significantly the total number 

 of nucleated marrow-cells per mg (Maisin, 1961). This increase occurs between 

 7 and 21 days after the administration of the dose. 



Knowlton and Hempehnann (1949) have shown that X-ray doses ranging 

 from 5 r to 325 r enhance the mitotic activity in the adrenal gland, 

 jejeunum, lymph node and epidermis of the mouse. They caU this pheno- 

 menon, overcompensation. It appears between a few hours and 10 days after 

 irradiation depending on the organ, and is more apparent in less radiosensitive 

 tissues. 



Pape and Jellinke (1950) and Pape (1951) stated that X-ray doses (1- 

 20 r) to the spleen led to cellular change and to enhanced resistance of mice to 

 whole-body X-radiation delivered later. He considered this protective action 

 as a cellular resistance of various tissues and due to prohferation of lymphoid 

 and reticulo-endothelial tissue originating from the irradiated spleen. 



In the present paper, we describe the results obtained after whole- 

 body irradiation of 1 r or 0-1 r. We show that one X-irradiation of such low 

 dose is unable to induce radioresistance against the medullary syndrome 

 produced by a subsequent LDgojgo), although the number of nucleated 

 marrow-cells per mg and the erythropoiesis (as measured by ^^Fe incorporation 

 in the red cells) and number of reticulocytes in the peripheral blood are 

 statistically increased during a certain period. 



EXPERIMENTAL CONDITIONS 



The homozygous L-strain rats were male and at the beginning of the 

 experiment were 4 months old and weighed 130 to 145 g. They were housed 

 in pairs. The animals were distributed in series of 40; 20 of them receiving a 

 first irradiation of 1 r or 0-1 r which can be called the conditioning dose, 20 

 serving as controls. Later, all 20 animals of each series were irradiated with 

 500 r X-rays which is i LDgg^go). 



The different experimental conditions: conditioning dose, sublethal dose, 

 time-interval in days between the conditioning and sublethal doses, the 

 number of rats in each group on the day of the administration of the con- 

 ditioning dose and of the sublethal dose are reported in Tables I and II. The 

 sublethal dose was always administered on the same day to conditioned rats 

 and to the controls. 



The rats were fasted for 24 hours before and after exposure to the sub- 

 lethal dose. They were not fasted for the administration of 1 or 0-1 r. For all 

 the irradiation, we used a General Electric Maxitron-250 X-ray apparatus 

 operating at 250 kV, 25 mA; 0-25 mm Cu -f 1 mm Al filter. For the adminis- 

 tration of the LDgojgo), the anticathode to rat distance was 80 cm and the 



