Studies on the Regulation of Granulopoiesis

S ummary . Tritiated thymidine autoradiography was used to study the control of granulocyte production and release using the irradiated leg shielded mouse as an experimental model. Neutropenia resulted in a shortening of 24–36 hr in the mean transit time through the non‐proliferating granulocyte compartment. There was little difference between neutropenic and control animals in labelling indices of the cells in the proliferative granulocyte compartments, which suggests that the granulocytic hyperplasia observed in the neutropenic mice was predominantly due to differentiation of morphologically unrecognizable precursor cells rather than increased proliferation of morphologically recognizable cells. The persistence of a labelling index of approximately 60% in the myeloblast‐promyelocyte compartment 24 hr after injection suggests that these precursor cells were rapidly proliferating.