The binding of bromine to serum proteins first observed by Sklavenitis and Comar was investigated. High-specific-activity K82Br was injected into a rabbit, and serum obtained 1 day later. Approximately 0.5% of the total 82Br in the serum was found to be protein bound at this stage. The application of various separation methods (electrophoresis, bromide exchange, denaturing followed by desalting) showed that one third of the protein-bound bromine is loosely attached whereas two thirds are firmly bound. After partial and complete enzymatic hydrolysis the bromine was found in the amino acid fraction. Essentially no bromine uptake was observed in the serum lipids and in an isolated thyroxine fraction.