A large number of plants are potential causes of dermatitis. Eruptions from them vary considerably in appearance, but are usually vesicular and accompanied by marked edema. After previous exposure and sensitization to the active substance in the plants the typical dermatitis may result from contact. The onset is sudden, a few hours or days after exposure, with involvement of the face, legs, wrists, and hands. There is a tendency for the dermatitis to spread, especially in scratch marks, so that linear lesions are produced. Contrary to general belief, the contents of vesicles are not capable of producing new lesions. No transferable sensitizing anti-bodies have yet been found in the blood serum. They are purely hypothetical and are postulated as sessile cellular entities.