By CARL J. GESSLER, M.D. B ECAUSE of the current interest in agranulocytosis following thiouracil administration it was determined to study the blood and bone marrow in a series of 9 patients under treatment with this drug for hyperthyroidism. It is of interest that before treatment was instituted the total leukocyte count in 4 of the 9 cases was less than 4,000 per cu. mm., with a correspondingly low value of the absolute number of neutrophils (table i). Two cases had hemoglobin values under 70 per cent, with a color index of less than i.