When it is incompletely saturated with Fe, transferrin may exist in 4 molecular forms: apotransferrin, monoferric (A) transferrin (with Fe occupying only the A site of the protein), monoferric (B) transferrin and diferric transferrin. By combining electrophoresis in urea-polyacrylamide gels with crossed immunoelectrophoresis using specific antihuman transferrin anti-serum, it is possible to display and estimate the concentration of each of these 4 forms in normal human serum. The distribution of Fe between the binding sites of transferrin is neither random nor determined by the relative binding strengths of transferrin''s 2 sites. The more weakly binding and acid-labile B site of the protein is predominantly occupied.