WHEN silk fibroin is dissolved in cupri-ethylenediamine and the solution neutralized and dialysed through a ‘Cellophane’ sac against running water, an aqueous solution of fibroin is obtained. Coleman and Howitt1 have suggested that, in this water-soluble state, fibroin can exist in two forms, since at pH approximately 3.0, part is precipitated while part remains in solution. They postulated that the former part represents denatured protein in which the molecules of fibroin have an extended chain configuration, whereas that remaining in solution is in a ‘renatured’ form, with the molecules folded around ‘proline hinges’.