A patient with chronic myelocytic leukemia had a cyclic oscillation of blood neutrophils, eosinophils, monocytes, platelets, normoblasts, and reticulocytes but not of lymphocytes. The cycle interval was 53--69 days. Except for reticulocytes all other cells cycled with neutrophils. Plasma colony-stimulating factor (CSF) oscillated out of phase with neutrophils, suggesting that granulocytopoiesis is regulated through CSF by a feed-back mechanism. Plasma erythropoiesis-stimulating factor (ESF) also oscillated. ESF crests preceded or coincided with reticulocyte crests, indicating that the ESF elevation may have been responsible for the reticulocyte peaks. The relationship between neutrophils and reticulocytes and their oscillations with plasma CSF and ESF suggests that there is a common stem cell which differentiates along one cell line or the other depending upon the balance of regulatory stimuli. The fraction of blood neutrophilic precursors (myeloblasts, promyelocytes, and myelocytes) in DNA synthesis fluctuated with neutrophil level. The calculated generation time was shorter at the crests than at the troughs of the neutrophil cycles. This finding suggested that the rate of proliferation of the neutrophils changed periodically. This observation, along with a periodic increase in differentiation of the stem cell toward the neutrophilic cells, is the probable explanation of oscillation of the neutrophil count in the blood.