The writer's investigation was undertaken with the object of attempting to throw more light on the chemical nature of the poisonous substance found in _Rhus toxicodendron_. Soon after commencing work, however, it became apparent that the poison could be more intelligently studied if the substances associated with it in the plant were first identified; the scope of the work was therefore extended to an investigation of the other constituents of the plant, and it was hoped that a knowledge of the properties of these constituents would suggest a more economical way of getting the poison than the method of fractional precipitation. The crude material for this work was prepared by Messrs. Parke, Davis & Co., of Detroit, Mich., according to special instructions submitted to them: 67-1/2 pounds of fresh leaves and flowers of poison ivy were collected near Detroit and carefully inspected by a competent botanist. This material was thoroughly macerated and put into ten-liter bottles with ether. The mass was thoroughly shaken, water being added to make it more mobile. The ether was then separated off and the extraction was repeated three times in the same way to insure complete removal of the toxicodendrol. The ether extracts were combined, thoroughly dried with anhydrous sodium sulphate, and the ether was distilled off, the temperature being kept below 40 deg. C. during the entire distillation. The residue after the removal of the ether was a thick, black, tar-like mass, weighing 3 pounds 11 ounces. In extracting the plant, about twenty-four gallons of ether were used. It is a significant fact in regard to the volatility of the poison that during the process of preparing this material none of the employees engaged in the work were in any way affected, since proper precautions were taken and the utensils were handled with rubber gloves. The crude ether extract, which will be designated as the "original material," was shipped to Baltimore in August and was kept in a cool place until November when the investigation was begun. When the bottle was opened, there seemed to be an escape of a vapor, and a nauseating odor suggesting crushed green leaves pervaded the atmosphere. Some days later, irregular red patches appeared on the face though a mask of cotton cloth was worn during the work, and the hands were protected by rubber gloves. Assuming from Pfaff's work that this original material contained the non-volatile oil toxicodendrol, the first experiment was to try to distil it out under diminished pressure. For this purpose, an Anschuetze distilling bulb containing ten grams of the tar was connected with a vacuum pump. After a pressure of 2 mm. had been established the bulb was gradually heated in a bath of Wood's metal. Nothing distilled over. The material began to carbonize at a temperature of 140 deg. to 150 deg.