With the use of a 95 per cent ethyl alcoholic extract of Toxicodendron radicans, (L.), (Rhus radicans), it is possible to reproduce, in susceptible individuals, the vesicular lesion typical of “poison ivy” dermatitis. It is also possible to confer varying degrees of protection to such susceptible individuals by means of hypodermic injections of the same alcoholic extract. A description of this mode of treatment, with its results, is given here, together with a discussion of later refinements in the preparation of the extracts and in the technic of treatment. The extract, as first employed by us, was prepared in the following manner: fresh, whole leaves of Toxicodendron radicans, with all excess moisture removed, were placed in clean glass jars, and were covered with 95 per cent ethyl alcohol, in a proportion of 1 gram of leaf to 3 cc. of alcohol. The jars were then tightly capped, and placed in the ice box.