1. The intravenous administration of killed typhoid organisms to dogs results in the development of a severe leukopenia which is followed by a marked leukocytosis.
2. The myelopoietic response of dogs to typhoid injections is quite similar to the response following leukopheresis.
3. Leukocyte DNA-P32 studies indicate that in response to typhoid stimulation, as with leukopheresis, the bone marrow constitutes the main reservoir for granulocytes contributing to peripheral blood leukocytosis.
4. Evidence is presented that, under these experimental conditions, leukocytes, after once having left the vascular tree, are unable to re-enter in any significant numbers.
5. The use of typhoid vaccine to stimulate leukocytosis combined with the use of P32 to measure the fate of cells released from the marrow is presented as an accurate and reasonably simple method to measure certain aspects of myelopoiesis in the experimental animal.