The effect of ascorbic acid oxidation and of peroxide on salivary glycoproteins has been studied. No signs of depolymerization of the glycoproteins were observed as a result of treatment of whole saliva or the isolated glycoprotein fraction from human saliva with an ascorbic acid, hydrogen peroxide and cupric ion combination. Sialic acid was analyzed with two methods which gave different chromogens of the acid. The conclusion could be drawn from the results that the effect of ascorbic acid oxidation was a decarboxylation of sialic acid in salivary glycoproteins. This also destroys the bacteria-agglutinating activity of saliva. Full protection of the agglutinin activity is given by salivary peroxidase. It is suggested that one role of peroxidases in saliva is to protect biologically active proteins from the action of oxygen or its radicals.