The relationship between postmitotic marrow neutrophils (PMMN) and neutrophil increment in blood after an intravenous dose of 3 mg hydrocortisone/kg was studied in patients with normal-sized spleens and compared with splenectomized and splenomegalic patients. PMMN were quantified from the ferrokinetic measurement of the normoblast number and the PMMN/normoblast ratio in bone marrow biopsy sections. In 12 control patients with normal PMMN the increment was 3.50 +/- 1.13 X 10(9) neutrophils/liter. An excellent correlation was found between the number of PMMN and the maximal neutrophil increment (y = 826x - 1021, r = 0.93, p less than 0.001) among 24 patients with normal spleen size and a wide range of marrow cellularity. Significantly higher responses were observed in 10 splenectomized patients (y = 872x + 1429, r = 0.92, p less than 0.001). The two regression lines were shown to be parallel, indicating a diminution of the response by 2.5 X 10(9) neutrophils/liter in the presence of a normal spleen. In 11 hypersplenic patients the responses were further reduced and more variable. Peak neutrophilia occurred after median values of 2, 3, and 4 hr in the hypersplenic, the control, and the splenectomized group, respectively. These studies indicate that allowing for the different response curves neutrophil increments may be used as an index of PMMN in patients with normal spleen size and in splenectomized patients. They further suggest sequestration of the prematurely released cells by the spleen.