It has been known definitely since the time of Koch that certain essential oils have considerable disinfectant value. It has been assumed since his time also that substances which have disinfectant value when in watery solution are largely rendered inactive by solution in oil. This assumption arose as a wide generalization from what was actually observed by Koch in only one case, namely, that of in vegetable oils, such as olive oil or cotton seed oil. In this case the validity of the observation is undoubted, but there have been a very few practitioners who have believed that in spite of the test tube indications the best way to apply as a wound disinfectant is in solution in vegetable oil. The recent application by Dakin7 and his associates, of solutions of a water-insoluble oil-soluble substance (dichloramin-T) to the treatment of wounds indicates that from the practical point of view the question of the usefulness of disinfectants in oil is by no means closed. As a consequence of these general considerations, I undertook to study these questions anew. The recognized methods of determining quantitatively the antiseptic value of substances presuppose water solubility. The immiscibility of water and oil prevents the use of broth culture (or the suspension of bacteria in salt solution) in the testing of an oily substance, or of material which has been in contact with an oil. It is doubtless for this reason the physical difficulty of testing by standard methods that the many new antiseptic oils and oil-soluble antiseptics are accepted solely on the basis of clinical experience and not on that of known germicidal power. The value of such oils has not up to the present been expressed in terms of a known standard, comparable to the phenol coefficient of water-soluble antiseptics. Face to face with the necessity of deciding accurately the antiseptic power of a large number of oils the need for a preliminary investiga-