The observation by Warburg (1) that the amount of oxygen used by a slice of Flexner-Jobling rat carcinoma was less than that absorbed by such normal tissues as liver or kidney, has opened the way for the further study of the metabolism of tumor and other rapidly growing cells. The immature white blood ceils of myelogenous leucemia resemble somewhat those of the malignant neoplasms in their growth rate and behavior, and response to exposures to Roentgen rays. They serve as excellent material for the study of oxygen consumption because of their availability and the ease with which they can be manipulated in vitro.Patients with leucemia frequently have an elevated basal metabolic rate, particularly when the blood contains many immature cells (Riddle and Sturgis (2)). The possibility of a causal relationship between the rate of oxygen absorption by the patient and the degree of maturation of the blood cells presents a problem toward the solution of which this study of cell respiration in the microspirometer offers some data.The studies reported here include observations on the blood of normal individuals, a patient with a leucocytosis from sepsis, and patients with chronic myelogenous leucemia in various stages of the disease.