disclosed is an economical method for the preparation of chondroitin sulfates a and c useful as an effective ingredient of medicaments from fish scales as a waste material discharged from fishery in large quantities . fish scales are enzymatically decomposed in an aqueous medium in the presence of a protease to isolate the chondroitin sulfate compounds and by - product polypeptides followed by removal of the by - product polypeptides from the aqueous solution by a cation - exchange treatment and then the aqueous solution of the chondroitin sulfate compounds is subjected to fractional precipitation by the addition of ethyl alcohol as the precipitant .