The clearance of radioiron from plasma and its appearance in circulating erythrocytes in normal subjects are studied. The importance of correcting for plasma iron fluctuations and for mean body hematocrit is illustrated. The data are analyzed by probability theory to determine relationships between intravascular and extravascular iron. Two refluxes are described, one of about 7 particles of every 100 leaving the plasma, and the second of about 23. The return times of these are about 5 hours and 8 days, respectively.
