This article is only available in the PDF format. Download the PDF to view the article, as well as its associated figures and tables. To the Editor.— Costusroot oil derived from Saussurea lappa of the chrysanthemum (Compositae family) was used for patch testing six individuals who were known to be contact-sensitive to plants of this family and to their sesquiterpene lactones. The oil, 1% in petrolatum, produced strong positive reactions when applied in five of the six individuals who were tested, and it produced negative reactions in five controls. Contact sensitivity to costusroot oil thus accompanies contact sensitivity to ragweed (Ambrosia), pyrethrum (Chrysanthemum), marsh-elder (Iva), bitterweed (Artemisia), sneezeweed (Helenium), false ragweed (Franseria), tansy (Tanacetum), feverfew (Parthenium), etc, as well as some edible composite plants, chicory (Cichorium), artichoke (Cynara), lettuce (Lactuca), and the garden chrysanthemum. Various degrees of contact sensitivity to these plants of the Compositae family and also to plants of the Magnoliaceae family, viz, Magnolia and the tulip tree (Liriodendron tulipi fera) and to sesquiterpene lactones derived from these plants have been