Physiology of granulocyte turnover translated to a model. We consider mitotic neutrophil precursors as a single pool, Np (proliferating neutrophils). The Np pool consists mainly of myelocytes, promyelocytes, and myeloblasts but also contains earlier precursors. Cells in the Np population proliferate at a mean rate p. After the last mitosis, cells enter the postmitotic maturation/transit pool at a rate q. Transit neutrophils remain in the postmitotic pool for a period of 4 to 6 days,^5,16,17  referred to as the transit time (Δ), before egressing from bone marrow into the blood pool. Most egressing cells are segmented neutrophils, but some cells leave as band neutrophils. In the blood, neutrophils exist both in a freely circulating pool and a marginal pool (cells retained in proximity to the endothelium). Because the circulating and marginal pool are considered to be in rapid dynamic equilibrium,^7,8  we consider them a single kinetically homogeneous pool, N_B. Blood neutrophils are lost from the circulation at a rate z primarily to the bone marrow, liver, and spleen.^18-20  This loss is generally considered to be a random, irreversible process.^5,8,17,21,22  GMP, granulocyte-monocyte progenitor cells; HSC, hematopoietic stem cell.