DETERMINATIONS of the concentration of protein-bound iodine (PBI) in serum are being widely used as a measure of thyroid activity. Although the normal range is generally accepted as 4–8 micrograms per 100 ml. of serum, no information is available as to the normal fluctuation of the PBI is one individual from day to day. If great reliance is to be placed on a single PBI result, then it is of importance that the circulating level of the PBI remain fairly constant. Danowski et al. (1) studied the PBI in 4 females and 2 males at about weekly intervals for one and a half to two months. Variations in the females ranged from 0.5 to 0.8 microgram per 100 ml. and in the males from 0.3 to 1.1 microgram per 100 ml. The authors concluded that the PBI level appeared to be relatively constant over a period of weeks in healthy young adults. MATERIALS AND METHODS The subjects were 2 males and 3 females. All the females were in the reproductive period, had normal regular menses, and were studied for slightly more than one complete menstrual cycle.