ABSTRACT: 1. A solution of Bombyx mori silk fibroin was digested with chymotrypsin. Amino acid analyses of the chymotryptic precipitate showed in addition to the main constituents Gly, Ala, Ser and Tyr, very small amounts of Lys, His, Arg, Asp, Thr, Glu, Pro, Cys, Val, Met, Ile, Leu, Phe and Trp. 2. A stable solution of the chymotryptic precipitate in 6m-urea was obtained by dialysing a solution in 50% (w/v) lithium thiocyanate against 6m-urea. 3. The dinitrophenylated chymotryptic precipitate in 6m-urea was fractionated by gel filtration and by ion-exchange chromatography. On Dowex 1 (X2), a main fraction I(d) and three further fractions with different amino acid compositions and molecular weights were obtained. 4. Specific rearrangement and fission of the bonds involving the serine nitrogen atoms of fraction I(d) and fractionation of the resulting mixture by gel filtration yielded five fractions. Two of these fractions had the compositions DNP-Ser-(Gly(6),Ala(4),Ser) and DNP-Ser-(Gly(4),Ala(2) or Ala(3),Ser) and are presumably double repeating units according to the proposed formula of Lucas, Shaw & Smith (1957), namely [Ser-Gly-(Ala-Gly)(n)](2), for n values of 2 and 1 respectively.
