Summary.Human spleen ferritin was labelled with131I and injected into two normal men. The labelled ferritin left the plasma rapidly. The experimental clearance curve could be fitted with accuracy into two single exponential functions. The first component,T1/2= 9 min, accounted for the clearance of about 90 of the labelled ferritin. Surface counting showed uptake of131I by the liver but not by the spleen. Such a rapid plasma turnover is similar to that found after injection of tissue ferritins into experimental animals but contrasts with the slow turnover previously found for131I‐labelled human plasma ferritin. Differential clearance of isoferritins from the plasma is an important factor explaining the biochemical and immunological differences between tissue and plasma ferritin
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