In order to study the effectiveness with which the proteins of one food supplement those of another it is essential that diets be planned so as to be highly satisfactory with respect to all other factors besides protein. The natures of the deficiencies of all the foods studied must first be well understood, so that any mixtures employed in experiments designed to reveal the biological values of combinations of proteins may be supplemented with respect to inorganic salts, the essential vitamins, etc. Under such considerations the success or failure of growth, reproduction or other function in the animals will turn solely on the quality of the proteins of the experimental diet. In Chapter VI (The Dietary Properties Of Individual Food-Stuffs. 120. Results Obtained With The Biological Method For The Analysis Of A Food-Stuff) the dietary properties of many of the more important of our foods are given in detail, so that it is now possible to appreciate the method employed in the study of the protein values of combinations of these articles of diet.