In order to develop new theories for desensitization, a potential skin sensitizer, 1, with two different haptenic ends, a pyrocatechol group and an alpha-methylene-gamma-butyrolactone moiety, separated by a straight nine-carbon-atom chain has been prepared and used to sensitize guinea pigs. A "monohapten" 8 containing an electrophylic alpha-methylene-gamma-butyrolactone, connected to a dimethoxybenzene group, has also been prepared. Both the "bihapten" 1 and the "monohapten" 8 were shown to be sensitizers. Bihapten 1 sensitized animals recognized only bihapten 1 (and not the lactone 8) while monohapten 8 sensitized guinea pigs reacted to both 8 and 1. Bihapten 1 treated animals were further sensitized to monohapten 8 and challenged to the latter: the skin intensity reaction was significantly lower than the test to 8 in 8-sensitized animals. Bihapten 1 seems therefore to "tolerize" against the alpha-methylene-gamma-butyrolactone end.