Serum transferrin is a mammalian iron-transport protein. It has two specific metal-binding sites that bind a variety of metal ions in addition to ferric ion. Equilibrium constants for the binding of zinc(II) have been determined by difference UV titrations using nitrilotriacetic acid and triethylenetetramine as competing ligands. The values are log K1* = 7.8 and log K2* = 6.4 in 0.10 M N-(2-hydroxyethyl)piperazine-N'-2-ethanesulfonic acid and 15 mM bicarbonate, pH 7.4 at 25 degrees C. Titrations of the two forms of monoferric transferrin show that K1* corresponds to zinc binding to the C-terminal site and K2* corresponds to binding at the N-terminal site. These results indicate that at serum bicarbonate concentrations, transferrin should have a higher affinity for zinc(II) than serum albumin and therefore could play some role in zinc transport. A linear free-energy relationship has been constructed which relates the formation constants of a series of zinc(II) and iron(II) complexes. On the basis of the zinc-transferrin binding constants, this relationship has been used to estimate an iron(II)-transferrin binding constant of 10(7.4). Using this ferrous constant and literature values for the ferric transferrin binding constant, one calculates a ferric transferrin reduction potential of -140 mV, which is easily within the range of physiological reductants. Such a result tends to support mechanisms for iron removal from transferrin in which the ferric ion is reduced to the less tightly bound ferrous ion.