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When blood plasma proteins are depleted by bleeding with return of the washed red blood cells (plasmapheresis) it is possible to bring dogs to a steady state of hypoproteinemia and a uniform plasma protein production on a basal low protein diet. These dogs are clinically normal but their resistance to infection is distinctly below normal. Introduction of variables into this standardized existence gives information relative to plasma protein production. Plasma protein production under these conditions with a plasma protein concentration of 3.5 to 4.2 gm. per cent is relatively constant. As the plasma protein concentration rises the plasma protein removed falls rapidly (Table 1). At 4.6 gm. per cent the protein removed is less than 50 per cent of the amount removed at a plasma protein level of 4.0 gm. per cent. Cystine appears to be an important amino acid for plasma protein formation. This shows in Table 2 and is supported by data coming from published experiments. These experiments related to the factors which control plasma protein production bear on the problems of shock, hemorrhage, and protein wastage and their treatment by plasma injections which hold the attention of surgeons and physiologists at the moment. Again we would emphasize the fluidity of body protein including plasma protein-an ebb and flow between protein depots and plasma protein-a "dynamic equilibrium" of body protein. A discussion of the passage of large protein molecules through cell borders is submitted.

When blood plasma proteins are depleted by bleeding with return of the washed red cells (plasmapheresis) it is possible to bring dogs to a steady state of hypoproteinemia and a uniform plasma protein production on a basal diet limited in protein. Such dogs are clinically normal but have a lowered resistance to infection and certain intoxications. Casein digests given by vein or subcutaneously to such plasma depleted dogs are effective in promoting abundant new plasma protein production. Casein digest L by vein is equivalent to whole liver of like protein equivalence by mouth. The ratio of new plasma protein production to protein intake is 20 to 25 per cent in both instances. Casein digest L by vein gives the same response in plasma protein output as the same digest by mouth. Protein digest X by vein requires addition of tryptophane and cysteine to be effective in plasma protein production. The added cysteine sulfur is more than 95 per cent retained by the dog. The speed of digest injection has no effect on its utilization, within the range tested. Casein digest L given by vein to non-depleted dogs is less well utilized than in dogs depleted of plasma protein.

Given healthy dogs, fed abundant iron and a limited protein diet, with sustained anemia due to simple bleeding, we can study the capacity of each animal to produce new hemoglobin and plasma protein. Some dogs can produce much hemoglobin and enough new plasma protein to maintain the plasma protein concentration at approximately a low normal level. It is probable that their plasma protein producing capacity is not fully extended (Table 2). Other dogs (Table 5) can produce the same amount of hemoglobin but a hypoproteinemia develops and continues which should mean a maximal stimulus to produce new plasma protein. In such dogs we have strong stimuli to produce simultaneously new hemoglobin and new plasma protein. The ratio of plasma protein to hemoglobin varies from 40 to 60 per cent. The total new formed blood protein may amount to 30 to 40 per cent of the total diet protein intake which shows that some dogs have remarkable capacity to conserve and use diet protein. In this emergency of simultaneous depletion of hemoglobin and plasma protein levels, the dog gives preference to hemoglobin manufacture no matter what one of the listed food proteins is tested.