The effect of xylitol on the activity of certain oxidoreductases of human granulocytes, lymphocytes, and erythrocytes was studied both in vivo and in vitro. The effect of xylitol consumption on certain chemical properties of serum and whole saliva was also investigated. Short-term peroral administration of 0.6 g xylitol or glucose per kg body weight and day had a similar effect on leucocyte peroxidase levels. 0.07-0.2 M xylitol and glucose did not differ in their effects on leucocyte peroxidase levels in vitro either. Peroral administration of xylitol increased the whole saliva lactoperoxidase levels compared to glucose administration, but the differences were not significant. The results suggested that although the consumption of xylitol may under certain circumstances increase the peroxidatic capacity of saliva, that of leucocytes is not significantly affected.