Several compounds containing the alpha-methylene-gamma-butyrolactone moiety have been tested on human volunteers and on guinea-pigs; the animals were experimentally sensitized by alantolactone, isoalantolactone and laurel oil. Of the two new lactones, spirolactone was the more reactive: this was confirmed by both animal and human testing. The synthetic lactones are less reactive than natural ones. alpha-Methylene-gamma-butyrolactone itself does not elicit cross-reactions in guinea pigs sensitive either to alantolactone or to isoalantolactone, or in patients sensitive to sesquiterpene lactones. The alpha-methylene-gamma-butyrolactone group is necessary for cross-reaction, but to be active, it has first to be substituted. It was also found that isoalantolactone, allegedly not allergenic, is in fact a sensitizer and cross-reacts with alantolactone. The cross-reaction between laurel and Frullania, found in man, also occurs in guinea-pigs. It is more evident when sesquiterpene lactone is the sensitizer and laurel used to elicit reaction.