It is common knowledge that certain individuals are more susceptible than others to the effects of poison ivy and such other plants as primrose and sumac. Great individual variations in the capacity to develop sensitiveness exist and there is considerable evidence to show that previous contact, its degree and its duration, is the important factor in making one person more sensitive than another. The introduction of the patch test has been an important contribution. In 1896, J. Jadassohn (1) studied a patient sensitive to mercury and observed that a piece of “gray plaster” 5 cm. square applied to the upper arm gave rise to a violent local reaction which appeared under the plaster in twenty-four hours and then spread rapidly over the whole body. In 1920, Markley (2) had a patient whose pet guinea pig caused a dermatitis of her breast and forearm.