Bovine serum albumin followed by radioiodine was introduced into the circulation of rats. After 24 hr the serum was examined to see if the exogenous protein had been labeled. Radioiodine-labeled bovine serum albumin was detected in the rat serum by 2 methods: gel filtration on Sephadex G 200 to show the formation of a labeled antigen/antibody complex after reacting the rat serum with rabbit anti-cow serum, and electrophoresis on cellulose acetate to separate labeled bovine serum albumin from rat serum proteins. The results show that in vivo radioiodine labeling of a circulating protein is feasible and suggest that the various albuminlike iodoproteins that have been found in the serum and thyroid of man and experimental animals may all be simply iodinated serum albumin. Evidence supporting this hypothesis and possible sites of serum protein iodination are discussed. (Endocrinology76: 570, 1965)

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