NATURAL silk is readily soluble in several solvents, such as cupriethylenediamine, lithium bromide and lithium rhodanide, phosphoric acid, etc. Generally, it is assumed that the dissolution is brought about in such a way that the solvent splits the hydrogen-bonds which intermolecularly connect adjacent fibroin chains, attaches itself to the positions now made available and thus transforms the fibroin into a hydrophilic; water-soluble modification. Schauenstein1 stated that this procedure could be followed by means of ultra-violet spectroscopy by virtue of the ‘peptenol’ chromophore, the intensity of which should be a measure of the intermolecular hydrogen-bonding, and which shows a distinct and specific absorption in the region of about 250 mµ. In the course of regeneration experiments with phosphoric acid solutions, different silk samples were investigated in a Beckman spectrophotometer, namely, natural silk, silk dissolved in phosphoric acid (diluted with water), the aqueous silk solution obtained by dialysing the phosphoric acid solution and, finally, films prepared by drying the aqueous solution. The absorption curves obtained are shown in Fig. 1. In view of the above considerations, this result would mean that natural silk contains by far the largest amount of intermolecular bonds, whereas the phosphoric acid solution has lost most of them. Yet the aqueous solution shows an increase in the hydrogen-bonding, and still more so does the film, although without reaching the high value of natural silk.