Thirty-five a-methylene-y-butyrolactones have been prepared and their allergenic properties tested on the skin of guinea pigs experimentally sensitized to (a) alantolactone (l),(b) isoalantolactone (2), and (c) a-methylene-ybutyrolactone (3). The two first groups of animals cross-react to lactones containing 9 to 18 carbon atoms but not to smaller a-methylene-y- butyrolactones. Conversely, animals sensitized to a-methylene-y-butyrolactone react only ...