Abstract: The present invention provides a process for stimulating the immune system of aquatic animals of the class Osteichthyes and subphylum Crustacea comprising administering an effective amount of a yeast cell wall glucan composed of glucopyranose units linked by predominately beta-1,3 glycosidic bonds, having at least one branch therefrom of glucopyranose units linked by beta-1,6 glycosidic bonds. Additionally the invention provides a process for enhancing the effect of vaccines by administering an effective amount of the described yeast cell wall glucan along with vaccine antigens. Further this invention also provides a process for obtaining a glucan particularly effective for stimulating the immune system of aquatic animals of the class Osteichthyes and subphylum Crustacea.