Polymorphism in primate thyroxine-binding prealbumin was investigated with agarose gel electrophoresis at pH 8.6. In the rhesus monkey (Macaca mulatta), three forms of this protein were found in random sera: a single rapidly migrating band similar to that in human and other primate sera, a single slowly migrating band cathodal to rhesus albumin, and a five-banded form, the most rapid and slowest bands of which corresponded to the other two forms. The frequencies of occurrence of these three forms were consistent with the hypotheses that rhesus prealbumin is under the control of two codominant autosomal alleles, PA(F) and PA(S), and that the protein occurs naturally in serum as a tetramer composed of similar subunits.It was possible, by simple mixing in vitro, to produce five-banded prealbumin patterns from rhesus PA SS serum and rhesus PA FF serum, M. arctoides serum, P. hamadryas serum, and human serum. Thyroxine was bound by all the hybrid molecules produced in this fashion.