The changes occuring in the blood with increasing age, birth to 21 days, in ddD strain mice were studied. The results obtained are as follows :
1. The mice showed rapid growth ; The average body weight of mice was 1.4 gm at birth and showed approximately seven-fold increase on the 21st day after birth.
2. Both of erythrocyte and leukocyte counts showed a mean values of 4.29 million and 2, 600 respectively at birth. The counts gradually increased according to the age, showing the mean values of 9.30 million for erythrocyte and 4, 100 for leukocyte on the 21st day after birth.
3. Reticulocyte counts showed a mean value of 440%‰ at birth, after which there was a tendency of decrease according to the age. It was 129‰ on the 21st day.
4. The most significant change through the body growth was in the differential count of leukocytes. At birth, the neutrophils had a mean of 82.2 per cent and the lymphocytes was 12.8 per cent. Within a short period, the number of lymphocytes increased rapidly, while that of neutrophils decreased, so that the percentage composition of those two cell types were reversed on the 5 th day after birth. On the other hand, myelocytes appeared very few until the 5th day after birth with a mean value of 0.6 per cent at birth. Metamyelocutes was observed in 3.1 per cent as a mean at birth and it was 0.7 per cent on the 21st day.
The changes observed in the number of eosinophils and monocytes were relatively mild and there was no significant variation in different age groups. Basophils were so rare that there was no need to be taken into consideration.

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