In the course of routine blood examinations made in Lane Hospital, San Francisco, it had been frequently noted that the polymorphonuclear leukocytes were below 60 per cent. in apparently normal individuals, and also that the lymphocytes ran over 40 per cent, in a sufficient number of apparently normal persons to make a differential count of somewhat doubtful value in the diagnosis of exophthalmic goiter, etc. In order to determine, if possible, the cause of these variations from the commonly accepted standard, the blood of one hundred normal individuals was examined. These were selected with a view to including outdoor as well as indoor workers, females as well as males, and ages between 21 and 50 ; in short, to approximate as nearly as possible, the average individual appearing in the clinics or hospital. Among those examined were physicians, students and nurses at Lane Hospital, soldiers at the Presidio and such