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 substance, 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.