Induction of allergic contact hypersensitivity to a sesquiterpene lactone, alantolactone, was studied in four strains of mice: C3H/He, DBA/2, Balb/b, and Balb/c. The last three were successfully sensitized. A significant dose/response was demonstrated in these species, as well as an experimental "overload effect" in Balb/c and Balb/b strains. Histologic studies confirmed the allergic nature of the reaction. From the overall results, alantolactone can be considered a moderate sensitizer in mouse as well as in guinea pig. This study shows that the murine model can be used for experimental contact sensitization with moderate allergens, without the use of Freund's adjuvant for induction.