Ten volunteers, previously sensitized to Costus Absolute, were exposed to 26 sesquiterpene lactones applied in concentrations of 1% in petrolatum, and seven of the ten men were additionally tested with several plant extracts known to contain sesquiterpene lactones. Eight of the ten volunteers had cross reactions with two or more of the purified chemicals, most notably with 8-deoxycumambrin, arbusculin C, arbusculin A, rothin A, and damsin. These chemicals are structurally related to lactones of Costus, and it seems likely that the major contact sensitizers in Costus are, in fact, sesquiterpene lactones. This view is supported by the positive reactions seen to some of the Compositae plant extracts. Further study is required to determine if plant and perfume dermatitis have a common clinical interrelationship based on sesquiterpene lactone cross-sensitivity.