Frullania spp. (Hepaticae, Jungermanniales) are agents of allergies. Extraction and fractionation of Frullania tamarisci has given, as the only allergenic component isolated, a levorotatory crystalline sutbstance, the structure of which is demonstrated. It is a sesquiterpene lactone. The racemic (+/-) form is isolated from a mixed sample of Frullania tamarisci and Frullania dilatata; the dextrorotatory form, from Frullania dilatata. The observed allergenic properties are shared by some other-sesquiterpene lactones.