1. The iron contents of a number of saliva samples were determined. The iron contents found varied from 0.10–0.61 γ per g of saliva, while in some cases the iron quantity present was so small that it could not be determined. 2. These iron contents are of the same quantitative order as the concentrations of this element in the various media in use for the preparation of diphtheria toxin. 3. Rosebeek claims to have shown the presence in various saliva samples of a number of free amino-acids which also occur in the diphtheria media used at present. 4. In view of these facts we thought it likely, that saliva generally would be a suitable medium forC. diphtheriae, in which it ought to be possible to show the formation of diphtheria toxin. 5. We have succeeded in demonstrating that in saliva samples which have been inoculated withC. diphtheriae and incubated, a toxin is formed. Upon intracutaneous injection into a guinea pig this toxin produces a characteristic local skin reaction. 6. This toxin is thermo-labile, is not neutralized by “normal” serum, but is neutralized specifically by anti-toxic diphtheria serum. So this toxin is diphtheria toxin. 7. Therefore saliva is a suitable medium forC. diphtheriae, which can, under certain conditions, form a comparatively strong diphtheria toxin.