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While attempting to obtain a dimensional, molecular picture of the living material of organisms, we found ourselves involved in the interrelationships which occur in protein-water mixtures. The water content of protoplasm in an actively growing period of the organism is about 85 per cent, and the protein content about 10 per cent; in a dormant period the water is greatly reduced, often to one-third or even less. This percentage of about 30 to 35 per cent water is also a critical region, experimentally, in the water content of many protein-water mixtures. Investigations of protein hydration have been carried out, to a great extent, with gelatin-water mixtures which have furnished a wealth of experiment and suggestions concerning these relations (38,39,40). Confirmation of these is beginning to come out of recent correlative investigations, among which infrared absorption and x-ray studies are playing an important part. When evidence from these physical methods is combined with that from chemical studies, a fairly complete story of gelatin-water relations is obtained.The gelatin molecule is a long polypeptide chain and, as such, may be considered as fairly typical of the primary structure of the simple proteins (41). A small portion of a typical protein chain model built to scale, using known interatomic distances and bond angles (46), is reproduced in figure 1. SUMMARYThree lines of investigation are brought to bear on gelatin hydration. (a) Agreement is shown between the number of coordinate water molecules potentially possible in a 35 per cent water-65 per cent protein system and the number computed to be present. (b) The location and distribution of the water molecules on the gelatin chain molecule is shown by x-ray analyses, (c) The manner in which the water molecules are bound to the protein chain is shown by infrared absorption»studies to be in all probability through hydrogen bridges.