Homologous S35-labeled albumin, gamma globulin, and alpha-beta globulin were transfused into rabbits and the specific activities of the electrophoretic fractions of the sera of the recipients were determined at various time intervals up to 12 days after injection. Detectable reincorporation into a fraction other than that transfused was found only in the gamma globulin fraction after albumin injection. This activity rose between 2 and 12 days and reached a level of 2 to 3 per cent of the extrapolated zero time activity of the albumin fraction. When homologous serum protein doubly labeled with I131 and S35 was transfused into mice, marked drops in the ratios of I131 to S35 in the serum and tissue proteins were observed between 1 and 48 hours after injection. On the basis of a determination of the absolute and relative amounts of I131 and S35 found in the various tissue and serum proteins, the amount of reincorporation of S35 into each protein was calculated. The relative amounts of reincorporation of S35 among the various tissues were remarkably similar to the relative amounts of incorporation of S35 after the injection of labeled free amino acids. It is concluded that serum protein does not form a major direct source of amino acids to the tissues but feeds them indirectly through the extracellular pool.

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