It is an object to provide polypeptide compositions originating from silk protein which have not only excellent cell growth-promoting activity associated with undegraded silk fibroin and sericin, the fibroin H-chain and L-chain, or the a component of sericin, but also excellent touch, extensibility, and the like inherent in fibroin and sericin having a molecular weight not higher than 200,000. A method for producing functional polypeptide compositions originating from silk protein having an average molecular weight not lower than 10,000 and not higher than 200,000 and being excellent for cell growth-promoting activity, extensibility and the like, comprising; solubilizing a raw silk protein material from the domesticated silkworm or Antheraea yamamai in an aqueous solution of neutral salt, the raw silk protein material having an average molecular weight larger than 200,000 and at least a part or the whole of the H-chain and L-chain of silk fibroin and sericin a left undegraded in the case of the raw silk protein material from the domesticated silkworm; treating subsequently the solution with a peptide bond-cleaving agent; and cleaving peptide bonds between specific amino acid residues of silk protein.