2. The ratio of (a) the amount of serum protein per week removed by bleeding above that regenerated by the dog when eating the protein-free diet, to (b) the dietary protein increment (i.e., above that required for nitrogen equilibrium) was termed the potency ratio. The results indicated that serum protein was slightly superior to casein and lactalbumin in promoting the regeneration of serum protein. However, the respective potency ratios, varying from approximately 0.51 to 0.36, were comparable and not widely divergent as those reported by others. It was concluded that, whereas in some individuals dietary proteins may be able to produce a significant increase in the serum protein concentration, the potency ratios are not sufficiently different to warrant the administration of any one protein in preference to another.

3. The inhibitory effect of the basal protein-free diet with respect to serum protein regeneration in the dog was also demonstrated by the inability of the protein concentration to attain the normal level in spite of discontinued plasmapheresis. However, a subsequent fasting period resulted in a progressive rise in the serum protein concentration until the normal value was approximated. These observations are interpreted as indicating that the products of tissue protein catabolism can be utilized in the formation of new serum protein.

4. The experimental production of what seems to be an inhibition of the serum protein regenerating mechanism was described. This observation together with the hypothetical evidence presented by Bloomfield (17) and Weech and his associates (9) suggests that the most profitable line of approach to solution of the problem of hypoproteinemia lies not so much in the evaluation of dietary factors but in finding a way for stimulating internally the serum protein regenerating mechanism, which seems to involve in some manner the capacity of the tissue to furnish protein for the needs of the plasma.

5. A hypothesis explaining the mechanisms responsible for serum protein formation was presented and the experimental support for it discussed. The rôle of tissue protein catabolism in this function was emphasized.