Fractionation of the fibroin of Bombyx mori with alkali.

2. If tryptic hydrolysis is carried out in a medium containing 6 % of lithium thiocyanate no precipitation occurs, and the hydrolysate may be acidified and dialysed to give a solution of peptides of high molecular weight. 3. Further hydrolysis of this fraction with chymotrypsin shows that it consists predominantly of long sequences of amino acid residues throughout which glycyl residues alternate with alanine, serine, valine and tyrosine residues. Such sequences make up almost 90 % of the fibroin molecule. 4. Amino acid analyses of the fractions obtained by enzymic hydrolysis are presented, and the distribution of the amino acids in fibroin is discussed.