The supposition that the glomerular filtrate is protein-free is based on the classical experimental studies of Wearn and Richards.1 They found glomerular fluid, obtained by micropuncture from Bowman's capsule, to be "protein free" or at least to contain less than 30 mg. per 100 ml. of protein, which was the least amount that could be detected by the methods at their disposal. As Addis2 has pointed out, this supposition might be readily accepted were it not that the volume of glomerular filtrate in man is approximately 180,000 ml. per 24 hours, so that even if the protein concentration was as little as 10 mg. per 100 ml., 18,000 mg. would pass through the membrane daily. Considerations of this sort stimulated workers in the field, so that during the past 10 years our knowledge of the passage of proteins through the kidney has advanced to the point where a