1. When the dog is subjected to quantitative plasmapheresis and fed appropriate "synthetic" artificial rations, it is possible to evaluate the ability of the organism to regenerate serum protein from both exogenous and endogenous sources. Approximately 44 per cent of the protein, casein, fed in excess of the minimal amount needed normally to meet the general nitrogen requirements, is utilized for the formation of new serum protein. Under our experimental conditions the dog can regenerate each week solely from endogenous sources approximately 0.6 gm. of this blood protein per kilo of optimal body weight. This is equivalent to about 21 per cent of the total amount of serum protein normally present in the plasma.
2. When the dog is fed an adequate protein diet and is subjected to a régime of prolonged intensive plasmapheresis (period of 16 consecutive weeks), no impairment in the ability of the organism to regenerate serum protein from either exogenous or endogenous sources occurs. Under our conditions of experimentation the dog appears to be able to form each week an amount of the blood protein approximately equal to that normally present in the plasma. Because of this remarkable ability of the normal organism to regenerate rapidly serum protein over a relatively long period of time, it seems that loss of protein alone in conditions of Bright's disease cannot be the etiologic agent responsible for the persistence of the hypoproteinemia. An additional factor, the "specific" ability of such individuals to regenerate serum protein, must be taken into consideration.