The development of protein hydrolysates and of amino acid mixtures suitable for clinical use has been one of the most important advances in medicine during the past few years. No doubt these substances will remain indispensable therapeutic agents for a long time. However, indications that the protein hydrolysates and mixtures of the pure amino acid now available may not provide optimal protein nutrition are appearing. This point has been emphasized recently1 by the Rochester University investigators, who state that oral proteins are better than complete amino acid mixtures or protein hydrolysates for certain clinical purposes. These conclusions obviously raise the question whether there may exist in proteins some other factor than the known amino acids which is important in human nutrition.
In 1941 Woolley and his associates at the Rockefeller Institute reported2 that some as yet unidentified factor was present in casein and certain other proteins which had