1. To assess the effects of irradiation on erythrocytes in vivo, one group of rabbits was injected with radioiron-tagged erythrocytes immediately before, and a second group immediately after exposure to x-rays. A third control group received the tagged erythrocytes but no irradiation. There was a drop in red cell mass in both irradiated groups as compared with the controls, but there was no difference between the two irradiated groups, i.e., between the behavior of irradiated and nonirradiated erythrocytes in irradiated hosts. These findings indicate that there is no direct effect of irradiation in the median lethal range on erythrocytes in vivo.

2. The degree of anemia was studied as a function of irradiation dose. It requires nearly lethal doses of x-rays to cause a conspicuous anemia. In the fatally irradiated animals the drop in red cell mass and total circulating radioiron is precipitous; in those nonfatally irradiated the drop is gradual and relatively slight, even though the animals may have received the same dose.

3. Increase of specific organ activity of the liver and spleen of irradiated animals can be correlated with hemosiderin deposits and congestion. Deposition of hemosiderin is a consequence of irradiation, its degree increasing with the effectiveness of the irradiation.

4. The erythrocyte mass of rabbits obtained by two direct isotopic technics (P32 and Fe59) and one indirect technic (T-1824 dye) differ by constant ratios. The Fe59/P32 ratio is 0.89; the Fe59 values are regarded as correct. With the aid of conversion factors either technic can be used. If an indirect technic is employed and the erythrocyte mass is estimated on the basis of plasma volume measurements, it is necessary to calculate first the average body hematocrit. This is done by multiplying the large vessel hematocrit by a conversion factor, which is 0.83 in normal rabbits.

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