1. The effect of various percentages of protein (casein) in the diet on the serum protein concentration of the rat was studied. 2. With high protein rations (50 per cent or over) there is no drop at all of serum protein concentration, even though the diet is otherwise poorly balanced and inadequate as evidenced by weight loss. 3. With protein rations under 20 per cent the serum protein concentration falls promptly to a level of about 5.25-5.50 gm. per cent, where it is maintained regardless of whether the diet contains 1, 5, or 10 per cent of protein. Further depletion of the blood colloid concentration seems to meet with resistance on the part of the body.