During the last twenty years colloidal solutions of various metals have been largely used in therapeutics. Few investigations, however, have been made on their mode of action, and most of these have been made on mammals. As a contribution to the subject some experiments made under simpler conditions seem worthy of record. The reaction employed was the effect of an electrolyte-free silver sol on bacteria. Somewhat similar experiments have been made by Henri and coadjutors. These, which are referred to later, were unknown to me at the time of the investigation.