Was examined the potential for using tyrosinase to graft the polysaccharide chitosan onto silk fibroin. Mushroom tyrosinase was observed to catalyse the oxidation of tyrosine of silk fibroin. The amino acid composition of oxidized silk fibroin showed a decrease of tyrosine concentration. Chemical changes were observed by FT-IR spectroscopy. Tyrosinase converts the phenolic substrate into electrophilic o-quinones, the o-quinones undergo a subsequent non-enzymatic reaction with nucleophilic amino groups of chitosan. Using slightly acidic conditions (pH=6) it was possible to modify silk fibroin under homogeneous conditions. The FT-IR spectra provided evidence that chitosan was grafted onto silk fibroin.