A method of solubilizing and extracting proteins from by-products was tested. The raw materials used were finely homogenized and digested at 60(DEGREES)C and pH 10.5 for 30 to 120 minutes in the presence of 0.5% alkaline nonspecific bacterial proteases from Bacillus subtillis. The protein in solution was separated from nonsoluble and organic solvent soluble components by filtration or centrifugation. When desired, the proteinaceous solution was dried (preferably by spray drying). Raw materials that were test digested included keratin from turkey feathers, bovine skin collagen, shark waste, shrimp heads, whole squid, inedible chicken carcass, bovine blood plasma, slaughterhouse waste, cotton gin waste, Enteromorpha sp. (a marine alga), Batis sp. and Distycilus sp. (two halophytes), soybean meal, casein, and fibrinogen. With this method, plant proteins were 57.4% to 59.9% extractable and animal proteins were 75.8% to 93.0% extractable. The native protein hydrolyzed by the procedure was reduced to an average molecular weight of 10,000-15,000 daltons. Other changes characteristic of the digestion process were increased protein concentration and decreased ash concentration. Complementation of by-product proteins in Tetrahymena medium resulted in increased growth compared to Tetrahymena cultures using soy or casein as the sole protein source up to 1.25 times. Decreasing protein molecular weight resulted in decreased growth in Tetrahymena (up to 4 times). Shrimp fed hydrolyzed animal proteins grew only 37.6% to 54.8% as much as squid-fed shrimp controls. White leghorn chicks fed 40% protein as hydrolyzed by-product proteins grew as much as chicks fed a commercial-type milo-soy diet supplemented with methionine. Amino acids from smaller peptides were more rapidly absorbed and more completely incorporated into muscle mass by chicks than were larger peptides.