1. Recipient rats were observed following periods of circulatory mixing with normal or leukemic donor animals.

2. The normal rat tail appears to store leukocytes in its vascular channels, as evidenced by a progressively falling leukocyte count from a tail incision as the tail is "milked."

3. Comparison of irradiated rat tail and arterial blood leukocyte counts during and after cross-circulation revealed the tail values to be consistently higher than the simultaneous arterial levels. This observation excludes a major removal and destructive mechanism by some organ such as the lung as a major cause of the rapid decrease in arterial blood leukocyte levels noted in irradiated rats after the cross-circulation procedure.

4. Tail blood leukocyte counts of irradiated, cross-transfused rats remained higher than those of control irradiated rats for at least three days.

5. After cross-circulation with leukemic donor rats, leukemic cells were identified in the tail blood of normal rats for at least three days.

6. Prolonged observations revealed that 2 irradiated Sprague-Dawley strain recipients of leukemic cells from Sherman strain leukemic donors survived the severe radiation damage but developed leukemia. This observation suggests that transfused leukemic white blood cells protect against radiation damage.

7. Five normal Sherman strain rats developed leukemia after cross-circulation with leukemic Sherman donors. This incidence (50 per cent) of transmission to adult animals is much higher than has been reported by conventional methods of transfer.

8. The observations reported support the concept of viability of leukocytes transfused in rats by this method, and suggest that at least one site of noncirculating transfused leukocytes is within sluggish vascular channels.

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