The reaction of silk with a disulfide-containing crosslinking agent, i.e. bis($eta$-isocyanatoethyl) disulfide(BIED), was studied in an attempt to obtain disulfide-crosslinked silk. BIED was reacted with silk both monofunctionally and birunctionally to form branches and crosslinks, respectively. The sulfur content of BIED-treated silk was decreased by reduction with tri-n-butylphosphine, for one of two sulfur atoms in the branch was removed from silk by reductive fission of the disulfide linkage. The effect of treatment conditions on the reaction of silk with BIED in dimethylformamide was studied. Higher concentration of BIED and longer reaction time increased the amount of combined BIED. Crosslinking efficiency was about 42.4-57.4%.