Bone marrow cells, from both normal and cyclically neutropenic (CN) dogs, treated with CN or normal dog serum produced large numbers of leukocytic colonies in soft agar cultures. The largest number of colonies was formed in cultures treated with serum taken from CN dogs at the nadir of the neutropenia. The serum colony-stimulating activity was shown to be inversely related to the CN dog’s peripheral neutrophil counts, suggesting a negative feedback control of granulopoiesis.

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