A method for stimulating cell growth includes administrating to a candidate a sponge protein hydrolysate which is a product by acidic hydrolysis of sponge skeleton fibers (spongin fibers) obtained from refined sponges as the raw material. The product has (i) a molecular weight of less than 5,000, with light coloring, and (ii) a human cell growth stimulating activity. The above substances can be used in the form of food product materials, pet food materials, cosmetics or medicinal products to reduce wrinkles and protect the hair cuticles.