The interaction of Hg(II) with human blood serum was studied at physiological pH. Most of the Hg(II) was found to be associated with the proteins, and only a small fraction was associated with the low-molecular-weight substances in serum. Albumin is the major Hg(II)-binding protein (greater than or equal to 90%) in serum. Among the amino acids, L-cysteine has the highest affinity for Hg(II). In dialyzed serum having equimolar concentrations of Hg(II), albumin, and L-cysteine, the amount of Hg(II) found in the supernatant after ultracentrifugation was about 6--7%. There are preferential Hg(II)-binding sites on the albumin molecule. However, no significant change in the circular dichroism spectrum of albumin was detected until at least two equivalents of Hg(II) were present. Hg(II) can mediate the formation of the albumin dimer as well as a ternary complex of the type albumin-Hg(II)-L-cysteine. The latter presumably plays an important role in the transport of Hg(II) between blood and various tissues.