Profound transient neutropenia, followed by an overshoot of the neutrophil (N) count to an average of 2.5 ± 1.0 (mean ± 1 SD) times higher than average control N count, has been induced in seven normal subjects by reinfusion of heparinized blood that had been stagnated in a hemodialysis coil for 15 min. This was similar to the neutropenia-neutrophilia cycle occurring shortly after the initiation of hemodialysis in uremic patients. At the time of neutropenia, profound monocytopenia was also observed, but only a slight drop in lymphocyte count occurred. Neither monocytes nor lymphocytes subsequently recovered to higher than control values. A DF32P standard N survival procedure, performed in one subject with Hodgkin’s disease in complete unmaintained remission, showed that a large number of unlabeled N appeared during the overshoot, suggesting their marrow origin. Although in two of three attempts neutropenia was produced by reinfusion of blood through the coil without stagnation, no significant overshoot of N count following neutropenia occurred without the stagnation step. Simple phlebotomy and reinfusion of blood without the coil had no effect on the N count. This model may prove useful in studying the possible immediate regulation of the N count by marrow release of N and marrow N reserves in various patients.