THIS paper is a preliminary report of the results of studies of blood amino nitrogen levels after the ingestion of test feedings of protein and of acid hydrolysates of protein. The study was undertaken to learn whether there were any characteristics of the peripheral blood amino nitrogen curve after the ingestion of these test feedings that would make it possible to distinguish between the various nutritional diseases of infants. Particular attention has been given to this amino nitrogen response in infants with congenital cystic fibrosis of the pancreas. It was hoped that the procedure would give information of diagnostic value, as well as help in the evaluation of certain therapeutic measures in common use. The first part of this paper deals with the use of the procedure as a diagnostic method and the second with its use as a guide in the evaluation of therapy. There have been a number