Abstract The circulating erythrocytes of 14 patients with hookworm infection were marked with both radioactive chromium (Cr 51) and radioactive iron (Fe 59). Radioactivity from either isotope was measured in blood and feces with a pulse height analyzer. Radioactivity from Cr 51 was taken to indicate how much blood and iron was being lost through the hookworms, and radioactivity from Fe 59 indicated the amount of iron actually lost through the feces, while the difference between the two values gave the quantity of iron reabsorbed. Intestinal iron loss varied between 2.74 and 21.34 mg. per day, while fecal loss ranged from 1.80 to 16.24 mg. per day. On the average, 44.1 per cent of the intestinal iron lost was reabsorbed, with a range of 13.1 to 76.4.