A protein that binds tetraiodothyroacetic acid (tetrac) with high specificity 
has been detected in normal rabbit serum. Scatchard plots revealed the protein 
to have a principal binding site with both high capacity and high affinity for 
tetrac (KA 4.8 X 10(10) M-1. Binding of tetrac by the protein is partially 
inhibited by barbital. During polyacrylamide gel electrophoresis at pH 8.0, the 
tetrac binding protein has a mobility characteristic of a prealbumin. As judged 
from competitive binding studies, the protein also binds tetraiodothyropropionic 
acid (tetraprop) firmly, but less so than tetrac. The apparent affinity of the 
protein for the triiodinated analogues of tetrac and tetraprop is only about 1% 
that for tetrac, and that for thyroxine (T4) only 0.6% that for tetrac; 
3,5,3'-triiodothyronine is not bound at all. The protein can be utilized in a 
competitive protein binding assay for tetrac in human serum, after removal of 
cross-reacting T4.