Biochemically known mucopolysaccharides can rarely be identified unequivocally in specific tisse sites by histochemical methods. Moreover, many carbohydrate-rich substances that have been detected histochemically (in terms of their vic-glycol content and characteristic acid groups) cannot be related to mucopolysaccharides and glycoproteins known to biochemists. Accordingly, a terminology and classification is proposed for delineating mucosubstances in situ in terms of their histochemical properties.