DESPITE the considerable advances that have been made in our knowledge of protein structure, the problem of denaturation remains somewhat obscure. A general theory of the phenomenon has been developed, which, although rough in its definition and speculative in character, is acceptable with varying degrees of qualification to most protein chemists. This theory supposes the uncoiling of a polypeptide chain (see, for example, Astbury and Lomax1); but so far direct evidence in support of it has been meager and confused by experimental results that reflect chemical and physical reactions of secondary importance. These reactions may be avoided to a large extent by the examination of a protein of the greatest possible simplicity of structure. Silk fibroin, which is free from sulphur and contains a negligible number of amino- and carboxyl-groups, offers possibilities in this direction.